[求助]在两个相交的集合中,X is in either A or B,指的是哪一部分?

9. A financial investment club has ** members with investment either in bond or in stocks. ** don't invest in either of them. ** invest in stock. ** invest in bond. how many people invest in both?

AND ：
            (1/2)  * (C + D –A)

我的理解是A = invest in either bond or stock,这里A指的是要不就是投资BOND，要不就是投资STOCK，不包括both的情况的。 B = none of them, C = stock, D = bond,

E = Both stock and bond

A + E = C + D – E 所以，E = 1/2 * (C + D –A)

注：如果把invest in either bond or stock理解成at least invest in one of the products. 那答案就是
        

我有点糊涂了,到底in either a or b,指的哪一部分呢?