1. There are 11 women and 9 men in a certain club. If the club is to select a committee of 2 women and 2 men, how many different such committees are possible? 答案是1980 請問如何求出

2. Is the measure of one of the interior angles of quadrilateral ABCD equal to 60 degrees?

(1) Two of the interior angles of ABCD are right angles.

(2) The degree measure of angle ABC is twice the degree measure of angle BCD.

答案是C 但是我想請問 怎麼會有四邊形 有兩個內角九十度(不就一定是正方形 或長方形了嗎?!) 但是剩下兩個不是九十度呢?!

3. Three printing presses, R, S, and T, working together at their respective constant rates, can do a certain printing job in 4 hours. S and T, working together at their respective constant rates, can do the same job in 5 hours. How many hours would it take R, working alone at its constant rate, to do the same job?

答案是12 請問如何求出???

感謝高手指點!! 萬分感激