求教几道GWD数学题

1)What is the value of (x - y)4? (1) The product of x and y is 7. (2) x and y are integers 答案是C,我不明白

2) When tossed, a certain coin has equal probability of landing on either side. If the coin is tossed 3 times, what is the probability that it will land on the same side each time? 1/8,1/4,1/3,3/8,1/2 这个不是1/8吗,为什么说是1/4呢?

 3)In the decimal representation of x, where 0 < x < 1, is the tenths digit if x nonzero? (1) 16x is an integer. (2) 8x is an integer. 为什么答案是B呢

4) If x, y, and k are positive numbers such that (x/(x+y))(10) + (y/(x+y))(20) = k and if x < y, which of the following could be the value of k? 10,12,15,18,30 答案是１８．

我数学弱得很，有谁能帮忙解答吗．万分感谢！！！

还有这道题也是GWD里的,有没有快捷的算法啊或者公式?

If an integer n is to be chosen at random from the integers 1 to 96, inclusive, what is the probability that n(n + 1)(n + 2) will be divisible by 8?

答案是5/8.

要是就这样硬算要花太长时间了.