Help with a Singapore problem

[Karie]

Can you help me figure this out???

 

There were 600 children on a field. 30% of them were boys. After 5 teams of boys joined the children on the field, the percentage of the children who were boys increased to 40%. How many boys were there in the five teams altogether?

 

Thanks!

[Blossom'sGirl]

The number of boys has not increased 10%. The base changes. At first, it is 600. But then it changes to whatever the total number of children are now.

 

The key to this problem is that the number of girls does not change.

70% are girls, which is 460.

Then, 60% are girls, which iis still 460.

Using the unitary method taught in the textbook:

60% of total is 460

10% of total is 460/6 = 70

40% of total is 70 x 4 = 280

 

Since there were 180 boys, and there are now 280, there are 100 more boys.

 

You can't say that the percentage of boys increases by 10% in because the base changes. You start with 30% of 600, and increase the total by 280, and the number of boys is 40% of 880, not 40% of 600.

 

 

This is copied from the Singapore math forums. Once you register, there is a lot of help available.

[cbollin]

The key to this problem is that the number of girls does not change.

70% are girls' date=' which is 460.[/quote']

 

Tiny typo in that.... There are 420 girls. not 460. ;)

procedure is right.

 

(edit to add for clarity)

and another tiny typo. I think it should say:

and the number of boys is 40% of 700, not 40% of 600. The total increases by 100 boys not 280 boys, right?

 

 

 

-crystal

[Karie]

Thanks!!!!!!!!!!!1