Q35:

Guy’s net income equals his gross income minus his deductions. By what percent did Guy’s net income change on January 1, 1989, when both his gross income and his deductions increased?

(1) Guy’s gross income increased by 4 percent on January 1, 1989.

(2) Guy’s deductions increased by 15 percent on January 1, 1989.

为何答案是E? 两者相加不是能算出来吗?

if gross income = x, and deductions = y, then before 01/01/89 the net income is x - y; after 01/01/89, the net income is 1.04x - 1.15y, and we want to know by how many did the net income grew, since the answer depends on the values of x and y(plug in some numbers the results will be different), thus 1) and 2) are not sufficient to solve the problem;