Q1 In 1986 book value of a certain car was 2/3 of original purchase price, and in 1988 its book value was 1/2 of original purchase price. By what percent did the book value of this car decrease from 1986 to 1988?

A) 16 2/3% B) 25% C) 33 1/3% D) 50% E) 75% key: B

Q2 The number of coronary-bypass operations performed in th US increases from 13,000 in 1970 to 191,000 in 1983. What was the approximate percent increase in the number of coronary-bypass operations from 1970 to 1983?

A) 90% B) 140% C) 150% D) 1400% E) 1600% Key: D

这是PP3上模考2中的两道很简单的题，但是我去搞不清楚这种比较的百分比问题，谁应该做分母。例如第一道题将 1986年的2/3 做分母，而第二题用13000做分母。

Q3 Jeff earns $12 per hour, and John earns $10 per hour. Approximately how much less as a perscentage does John earn than Jeff per hour?

Ans: (12-10)/12 以Jeff 的每小时工资作为分母

不知道这种题型有没有规律可寻 ，如果说分子是差量，那么分母呢？请大家指点！