If a pre-school admits 15 additional boys to its current pop

[saryhamzah]

If a pre-school admits 15 additional boys to its current population, the percentage of girls will be 40%. If currently the number of boys equals the number of girls, how many total students are currently enrolled in the school?

A) 30
B) 40
C) 60
D) 90
E) 120

The above is a question from MGMAT FoM online homework questions - Foundations of Math I: 1.8 Bring It Together - Equations. I understand that an equation must be setup. The equation is as follows:

(b + 15)/(b + b + 15) = 3/5

I do understand that we are setting the equation up to the fractional equivalent of 60%. I do not understand why. Why can we not set it up to the fractional equivalent of 40%? 2/5? What is the logic behind setting the equation up to 3/5?

Thank you,

[RonPurewal]

If a pre-school admits 15 additional boys to its current population, the percentage of girls will be 40%. If currently the number of boys equals the number of girls, how many total students are currently enrolled in the school?

A) 30
B) 40
C) 60
D) 90
E) 120

The above is a question from MGMAT FoM online homework questions - Foundations of Math I: 1.8 Bring It Together - Equations. I understand that an equation must be setup. The equation is as follows:

(b + 15)/(b + b + 15) = 3/5

I do understand that we are setting the equation up to the fractional equivalent of 60%. I do not understand why. Why can we not set it up to the fractional equivalent of 40%? 2/5? What is the logic behind setting the equation up to 3/5?

Thank you,

if the population is 40% girls, then it's 60% boys.
your equation here is phrased in terms of boys, not girls, so it's 60%.

by the way, you don't need algebra to solve this problem; you can just work backward from the answer choices.
if you take (c), for instance, then there are currently 30 boys and 30 girls. if you add 15 boys, as stipulated, then you have 45 boys and 30 girls, 75 students total.
30/75 is 40%, so (c) is correct. done.