Question

A battery manufacturer randomly tests 500 batteries and finds that 3 are defective.

Answer 1

(3/500)*(12,000) = 72 batteries . . . answer is B) 72

Answer 2

If you set up the problem and put 500 / 3 = 12,000 / x, you could cross multiply. 12,000 times 3 = 500 times x. 
36,000 = 500x Now divide both sides by 500.
72 = x
The answer is B.72!