Apply the average formula here. We know the total divided by the number of things is the average. First figure out the total amount spent. $1.25 was spent on the cheaper marbles (5*$0.25). Let's say the number of pounds of expensive marbles are y, then the amount spent on expensive marbles is (0.32)* y. So the total spent is $1.25 +.32y. Now divide this by the number of things . In this case that is the number of pounds are 5 + y. We know that should give an average of about 0.29. So we can set up the following equation.

$$\frac{1.25\ +.32y}{5\ +\ y\ }=\ 29$$

Multiply both sides by 5 + y to get

1.25 +.32y = .29y + 1.45
.03y = .20
y = 20/3 = 6.666...

This is closes to 7, so the answer is C.

My concern on this one is if you plug in the answers and work with the numbers given. Choice B would have an average of 0.288888 and choice D would have an average of .2908333. One could make an argument that both of those values are approximately 29 cents since they both round to 29 cents. So the test writers would have a hard time justifying that there is only one answer here with the current wording.

Take care,
Sionainn

AAPL wrote:A boy is buying two kinds of marbles. One kind costs 25 cents per pound and he purchased 5 pounds of this kind. The other kind of marble costs 32 cents per pound. If the boy spent approximately 29 cents per pound for everything, how many pounds of the more expensive marbles did he buy?

A. 5
B. 6
C. 7
D. 8
E. 9

We can let n = the number of pounds of the more expensive marbles that he bought and create the equation:

(25 x 5 + 32 x n)/(n + 5) = 29

125 + 32n = 29n + 145

3n = 20

n = 20/3 = 6 2/3 ≈ 7

Answer: C