# HELP math question

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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

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You get a bigger number because what you are dividing it by is less than 1.:)

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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

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.

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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

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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.

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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

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.

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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!!!

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Both answers here are genius, pure genius!!!

:thumbup:

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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

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.

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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 by sheryl
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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.

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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.

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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.

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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 :)

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Both answers here are genius, pure genius!!!

:iagree:Sounds to me like Singapore Math. :)

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