Bunuel wrote:

Tough and Tricky questions: Mixture Problems.

An alloy weighing 24 ounces is 70 percent gold. How many ounces of pure gold must be added to create an alloy that is 90 percent gold?

A) 6

B) 9

C) 12

D) 24

E) 48

Initial: 24 ounces, 70 %gold or, 0.7*24 = 16.8 ounces gold and rest 7.2 other material.

Setp 1: Only gold is added.

Let x ounces gold is added.

Gold: 16.8 + x, Other material = x

Final: We need 90% gold in the alloy.

Total weight = 24 + x

16.8 + x should be 90% of 24 + x

Hence, 16.8 + x = 0.9*(24 + x)

0.1*x = 4.8

x = 48

Option E