What is x

[j_shreyans]

90 students represent x percent of the boys at Jones elementary school. If the boys as Jones elementary make up 40% of the total population of x students. What is x.

A)125
B)150
C)225
D)250
E)500

[GMATGuruNY]

90 students represent x percent of the boys at Jones elementary school. If the boys as Jones elementary make up 40% of the total population of x students. What is x.

A)125
B)150
C)225
D)250
E)500

We can PLUG IN THE ANSWERS, which represent the value of x.

D: x=250
The boys as Jones elementary make up 40% of the total population of x students.
Thus, the total number of boys = 40% of the 250 students = (0.4)(250) = 100.
90 students represent x percent of the boys at Jones elementary school.
x% of the boys = 250% of the 100 boys = (2.5)(100) = 250.
Since only 90 students should represent x% of the boys, a SMALLER ANSWER CHOICE is needed.
Eliminate D and E.

B: x=150
The boys as Jones elementary make up 40% of the total population of x students.
Thus, the total number of boys = 40% of the 150 students = (0.4)(150) = 60.
90 students represent x percent of the boys at Jones elementary school.
x% of the boys = 150% of the 60 boys = (1.5)(60) = 90.
Success!

The correct answer is B.

[Brent@GMATPrepNow]

90 students represent x percent of the boys at Jones elementary school. If the boys as Jones elementary make up 40% of the total population of x students. What is x.

A)125
B)150
C)225
D)250
E)500

Here's an algebraic solution:

90 students represent x percent of the boys at Jones elementary school
Let B = the number of boys at the school
So, we can write: 90 = x% of B
In other words, 90 = (x/100)B

The boys at Jones elementary make up 40% of the total population of x students
We can write: B = 40% of x
In other words, B = (40/100)x
Simplify to get: B = (2/5)x

Now take 90 = (x/100)B and replace B with (2/5)x to get...
90 = (x/100)(2/5)x
Simplify: 90 = 2x²/500
Multiply both sides by 500 to get: 45,000 = 2x²
Divide both sides by 2 to get: 22,500 = x²
Solve: x = 150

Answer: B

Cheers,
Brent

[Matt@VeritasPrep]

One last approach!

We know that

90 = x% of the boys

and that the boys = 40% of the total, or 40% of x,

so we have the equation

90 = x% of (40% of x)

or

90 = (x/100)(.4x)
90 = (.4x²)/100
9000 = .4x²
90000 = 4x²
22500 = x²
150 = x