Question

The ratio of the number of boys to the number of girls in a school is 5:7.If there are 600 students in the school,how many girls are there?

Answer 1

$5x- number\ of\ boys\\7x-\ number\ of\ girls\\\\ 5x+7x=600\\ 12x=600\ \ \ \ | divide\ by\ 12\\x=50\\\\ 5x=250\\7x=350\\\\ There\ are\ 250\ boys\ and\ 350\ girls.$

Answer 2

So,

The secret to solving problems with ratios is to find the value of one unit.

5:7 = 12 units total

To find one unit, divide the total number of students by the total number of units.
600/12 = a

Simplify
50/1 = a
50 = a

The value of each unit is 50.

Now, multiply the units by the numbers in the ratio.
50(5) = b
250 = boys

50(7) = x
350 = x

There are 350 girls.