3 variables 1 equation..tearing my hair out

This is a classic weighted average problem where X and y are two elements and X is weighted by 30 and Y is weighted by 60. we are asked to find the weighted average.

Wtd Avg (Z) = [(30*x) � (x+y)] + [(60*y) � (x+y)]

since Y is weighted more heavily, the wtd avg should be closer to Y's weight than X's weight. In this case z should be closer to 60 than 30. Only option that suggests so is 50.

if X and Y were weighted equally by say 50 each, z would have been 50
if x was given a 0 weight, and Y was given 70 weight, the wtd avg would be 70
If y was given 0 weight and X was given a weight of 40, wtd avg would be 40

however, usual practice in weighted average problems is that SUM of weights is equal to 1.

faraz_jeddah wrote: x and y are positive integers such that y>x. If 30* ( x / (x+y) ) + 60 ( y / (x+y) ) = z, what could be the value of z?

A)30
B)40
C)45
D)50
E)60

I believe that the added conditions (in red above) reflect the intention of the problem.

Putting the sum over a common denominator, we get:
(30x + 60y) / (x+y) = z.

Let x = the number of $30 shirts purchased at a certain store.
Let y = the number of $60 shirts purchased at a certain store.
Total cost of the $30 shirts = 30x.
Total cost of the $60 shirts = 60y,
Total number of shirts purchased = x+y.
Thus, the AVERAGE cost per shirt is equal to the following:

(30x + 60y) / (x+y).

In the problem above, z represents the average cost per shirt.
Since each shirt costs either $30 or $60, the average cost per shirt must be BETWEEN 30 and 60.
Since y>x, the number of $60 shirts purchased is GREATER than the number of $30 shirts purchased, with the result that the average cost per shirt must be CLOSER TO 60 than to 30.
Of the answer choices, the only viable option is z=50.

The correct answer is D.