tonygirl Posted September 11, 2012 Share Posted September 11, 2012 Can someone tell me how 20 divided by 4/5 is 25?? how can I get a bigger number then I started with? My mom thinks its 16 and the math test for my son gives both answers Help please Quote Link to comment Share on other sites More sharing options...

2girls4us Posted September 11, 2012 Share Posted September 11, 2012 You get a bigger number because what you are dividing it by is less than 1.:) Quote Link to comment Share on other sites More sharing options...

justasque Posted September 11, 2012 Share Posted September 11, 2012 (edited) Can someone tell me how 20 divided by 4/5 is 25?? how can I get a bigger number then I started with? My mom thinks its 16 and the math test for my son gives both answers Help please You have 20 pizzas. You're feeding teenage boys. Each boy will eat 4/5 of a pizza. (The pizza is cut into 5 slices, each boy will eat 4.) How many boys can you feed? How many groups of 4/5 are there in 20? Divide 4/5 into 20 (that is, divide 20 by 4/5). Answer is 25. Here's how: 20 divided by 4/5 Change 20 to a fraction: 20/1 divided by 4/5 Flip the second fraction and change the division to multiplication: 20/1 x 5/4 Multiply numerators and denominators: (20x5)/(1x4) = 100/4 = 25 There are 25 "4/5's of a pizza" servings in 20 pizzas. OR - 20 pizzas x 5 slices per pizza = 100 slices. Divide by 4 slices per serving = 100/4 = 25. You get a bigger number because what you are dividing it by is less than 1.:) Exactly. Because each boy is not eating a whole pizza, so there is some leftover from each pizza to go towards feeding another boy, so 20 pizzas can feed more than 20 boys. Each pizza can feed one boy (20 boys) plus the leftovers from 4 pizzas can together feed one boy, so the leftovers from 20 pizzas can feed 5 boys (20/4=5), so you can feed 5 more boys with the leftovers. 20 + 5 = 25 boys total. Edited September 11, 2012 by askPauline Quote Link to comment Share on other sites More sharing options...

ReneTL Posted September 11, 2012 Share Posted September 11, 2012 I'm sure there will be more sophisticated answers, but think of it this way: if you divide 20 into individual groups of 1, you would get 20. If you divide 20 into individual groups of 2, you would get 10; the groups get smaller as you go up. Now if you divide 20 into individual groups of 4/5, you would have to have more than 20, right? The rule for dividing by a fraction is to multiply by its reciprocal, so the equation is 20*5/4. HTH. Rene Quote Link to comment Share on other sites More sharing options...

Perry Posted September 11, 2012 Share Posted September 11, 2012 The easiest way to think about this is using a ruler or number line. Lets say you are dividing 12 by 2. You are really asking "how many 2s (or groups of 2 inches) are there in 12?" You know there are 6, because 6*2=12. Now, ask how many 1/2s there are in 12 (i.e. how many 1/2 inches in 12 inches). There are 24. Quote Link to comment Share on other sites More sharing options...

Mrs Mungo Posted September 11, 2012 Share Posted September 11, 2012 Can someone tell me how 20 divided by 4/5 is 25?? how can I get a bigger number then I started with? I think the pizza from askPauline and the ruler answer from Perry are both pretty good explanations. Don't think I can do better than those. My mom thinks its 16 and the math test for my son gives both answers Help please What do you mean that the math test gives both answers? When you divide fractions, then you multiply by the inverse. That means multiplying 20 times five (100), then dividing by 4, which would equal 25. Fractions are always a division problem. Quote Link to comment Share on other sites More sharing options...

Twigs Posted September 11, 2012 Share Posted September 11, 2012 You have 20 pizzas. You're feeding teenage boys. Each boy will eat 4/5 of a pizza. (The pizza is cut into 5 slices, each boy will eat 4.) How many boys can you feed? How many groups of 4/5 are there in 20? Divide 4/5 into 20 (that is, divide 20 by 4/5). Answer is 25. Here's how: 20 divided by 4/5 Change 20 to a fraction: 20/1 divided by 4/5 Flip the second fraction and change the division to multiplication: 20/1 x 5/4 Multiply numerators and denominators: (20x5)/(1x4) = 100/4 = 25 There are 25 "4/5's of a pizza" servings in 20 pizzas. OR - 20 pizzas x 5 slices per pizza = 100 slices. Divide by 4 slices per serving = 100/4 = 25. Exactly. Because each boy is not eating a whole pizza, so there is some leftover from each pizza to go towards feeding another boy, so 20 pizzas can feed more than 20 boys. Each pizza can feed one boy (20 boys) plus the leftovers from 4 pizzas can together feed one boy, so the leftovers from 20 pizzas can feed 5 boys (20/4=5), so you can feed 5 more boys with the leftovers. 20 + 5 = 25 boys total. The easiest way to think about this is using a ruler or number line. Lets say you are dividing 12 by 2. You are really asking "how many 2s (or groups of 2 inches) are there in 12?" You know there are 6, because 6*2=12. Now, ask how many 1/2s there are in 12 (i.e. how many 1/2 inches in 12 inches). There are 24. Both answers here are genius, pure genius!!! Quote Link to comment Share on other sites More sharing options...

justasque Posted September 11, 2012 Share Posted September 11, 2012 Both answers here are genius, pure genius!!! :thumbup: Quote Link to comment Share on other sites More sharing options...

Clear Creek Posted September 11, 2012 Share Posted September 11, 2012 Can someone tell me how 20 divided by 4/5 is 25?? how can I get a bigger number then I started with? My mom thinks its 16 and the math test for my son gives both answers Help please 4/5 of 20 = 4/5x20 or 20x4/5 (which is what you are thinking the problem means). The key to solving this type of problem is to remember that "of" means "times." The problem is asking 20Ã·4/5, which is different. It is asking how many 4/5 you can take from 20. Since 4/5 is less than one, you will be able to take out more than 20 of them from 20. Quote Link to comment Share on other sites More sharing options...

sheryl Posted September 11, 2012 Share Posted September 11, 2012 (edited) No, it's not 25. 4/5 of 20 = 16 4/5 x ?/? = ?/20 How many 5's go into 20? 4 Now you convert to "golden one" 4/4 4/5 x 4/4 = 16/20 HTH! I think I have this wrong....backwards....oh well. Edited September 11, 2012 by sheryl Quote Link to comment Share on other sites More sharing options...

Perry Posted September 11, 2012 Share Posted September 11, 2012 No, it's not 25. 4/5 of 20 = 16 4/5 x ?/? = ?/20 How many 5's go into 20? 4 Now you convert to "golden one" 4/4 4/5 x 4/4 = 16/20 HTH! 20 divided by 4/5 is not the same as 4/5 of 20. As to the rest of it, I'm not sure what you're doing. Quote Link to comment Share on other sites More sharing options...

Clear Creek Posted September 11, 2012 Share Posted September 11, 2012 No, it's not 25. 4/5 of 20 = 16 4/5 x ?/? = ?/20 How many 5's go into 20? 4 Now you convert to "golden one" 4/4 4/5 x 4/4 = 16/20 HTH! The problem isn't 20 times 4/5, it is 20 divided by 4/5. That is where the confusion lies. You and the OP are trying to multiply it, whereas the original problem stated that it was divided. Quote Link to comment Share on other sites More sharing options...

sheryl Posted September 11, 2012 Share Posted September 11, 2012 20 divided by 4/5 is not the same as 4/5 of 20. As to the rest of it, I'm not sure what you're doing. Hi Dear, I just corrected myself. Quote Link to comment Share on other sites More sharing options...

tonygirl Posted September 12, 2012 Author Share Posted September 12, 2012 I think the pizza from askPauline and the ruler answer from Perry are both pretty good explanations. Don't think I can do better than those. What do you mean that the math test gives both answers? When you divide fractions, then you multiply by the inverse. That means multiplying 20 times five (100), then dividing by 4, which would equal 25. Fractions are always a division problem. The is multiple choice they give you 3 options and the last options is non of the above thats what I mean but it giving both answers :) Quote Link to comment Share on other sites More sharing options...

Susan in KY Posted September 12, 2012 Share Posted September 12, 2012 Both answers here are genius, pure genius!!! :iagree:Sounds to me like Singapore Math. :) Quote Link to comment Share on other sites More sharing options...

## Recommended Posts

## Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.