prep math

求教数学题目：

1. Bobby bought two shares of stock, which he sold for $96 each, if he had a profit of 20% on the sale of one of the shares but a loss of 20% on the sales of the other share, then on the share of both shares combined, Bobby had:

a. a profit of $10, b. a profit of $8, c a loss of $8, d. a loss of $10, e. neither a profit nor loss

答案是：C， 请问怎么考虑这个题目的？

2. If positive integer j divisible by a great number of different prime numbers than the positive integer k?

(I) j is divisible by 30,

(II)k=1,000

Data sufficiency题目答案是：both statement together are sufficient, but neither statement alone is sufficient.

为啥不是None is sufficient?

我考虑是j=2*3*5*X,  k=2的9次方为最多prime numbers, 但是j是未知的，所以不能得出是否有更多的prime number,觉得我的考虑是否错误了？

3. If k and t are integers and k的平方-t的平方 is odd integer, which of the following must be an even integer?

(I) k+t+2

(II) k的平方+2kt+t的平方

(III)k的平方+t的平方

a.None, b. I only c. II only d. III only e. I, II, III

答案是：A， 请问怎么考虑奇数偶数的题目？谢谢！