单选题
【正确答案】
A
【答案解析】Note that, when a number is decreased by 20%, what remains is 80% of the original: 80% of 80% of 80% = .8 × .8 × .8 = .512 = 51.2%, which is greater than 50%. Column A is greater.
** Use TACTIC 2, Chapter 12. Assume that in 1970 the population of Mayberry was 100. Then, in 1980 it was 80, in 1990 it was 64 (80% of 80), and in 2000 it was 51.2, which is greater than 50, one-half of the 1970 population.
** Use TACTIC 2, Chapter 12. Assume that in 1970 the population of Mayberry was 100. Then, in 1980 it was 80, in 1990 it was 64 (80% of 80), and in 2000 it was 51.2, which is greater than 50, one-half of the 1970 population.
【正确答案】
C
【答案解析】Since the entire region is a circle of radius 4, its area is π(4)2 = 16π. The white region is a circle of radius 3 minus the small shaded circle of radius 1, so its area is 9π -π = 8π.
Thus, the area of the white region is one-half of the total area, and the area of the shaded region is the other half. The columns are equal (C).
Thus, the area of the white region is one-half of the total area, and the area of the shaded region is the other half. The columns are equal (C).
【正确答案】
D
【答案解析】Use TACTIC 4, Chapter 12. Could the aver-age of a and b equal 33? Only if the sum of a and b were 66. Is that possible? Yes, if a = 30.5 and b = 35.5. Must a + b = 66? Of course not. Neither column is always greater, and the two columns are not always equal (D).