Question

A girl bought mangoes at 7.50 each. There were two types of mangoes: good ones and not so good ones. She sold the good ones at 7.90 each and the not-so-good ones at 7.70 each. If she earned 34, how many mangoes did she buy?

Answer 1

number of good ones --- x
number of not-goods --- y

cost = 7.5x + 7.5y

return = 7.90x + 7.70y

7.9x + 7.7y - 7.5x - 7.5y = 34
.4x + .2y = 34
2x + y = 170

y = 170-2x

There is no unique answer, unless you supply more information.
I could pick any value of x between 0 and 85, and have a solution

e.g. let x = 50, then y = 70
cost = 7.50(50+70) = 900
revenue = 7.9(50) + 7.7(70) = 934
for a profit of $34

let x = 10, then y = 150
cost = 7.5(160) = 1200
revenue = 7.9(10) + 7.7(150) = 1234
for a profit of $34

etc