趣味的數學-59

抽屜里放有紅襪子和黑襪子2種顏色的襪子。隨機抽取2隻襪子時，抽出2隻襪子都是紅色的概率是1/2。

（a）抽屜里的襪子數量至少有幾隻？

（b）如果黑色襪子的數量是偶數，抽屜里的襪子數量至少又有幾隻？

【原文】

A drawer contains red socks and black socks. When 2 socks are drawn at random, the probability that both are red is 1/2.

(a) How small can the number of socks in the drawer be?

(b) How small if the number of black socks is even?