1. If a and b are both odd numbers, then which of the following must be an odd number ?   2a + b + 1   a + b   a + b + 2   ab + 1   ab   Hint Half-n-half Clue   2. If x + y = 15 and 5 x - y = 27, then x - y is equal to   15   -33   1   0   -1   Half-n-half Clue   3. If the base of a triangle is decreased by 20%, and its height is increased by 20%, then the area of the triangle   decreases by 35%   decreases by 4%   decreases by 20%   remains unchanged   increases by 20%   Half-n-half Clue   4. A particular model of an air-conditioner cost $ 800 five years ago. The same model costs $ 960 today. By what percent has the cost increased?   20   109   9   120   108   Half-n-half Clue