If a and b are two non-zero numbers and if 69(a+b) = (69+a)b, than which of the following must be true?

(A) a=69
(B) a+b=ab
(C) b=69
(D) a<1

Answer: (C) b= 69

Explanation:

69(a+b) = 69a+69b (by descriptive law)
(69+a)b = 69b+ ab

then,
69a+69b = 69b + ab
69a = ab
b = 69 (dividing both side by a)