请教一个prep的math问题

题目如下：In a stack of boards at a lumber yard, the 20th board counting from the top of the stack is immediately below the 16th board counting from the bottom of the stack.  How many boards are in the stack?

(A) 38

(B) 36

(C) 35

(D) 34

(E) 32

还有一道是：For every positive even integer n, the function h(n) is defined to be the product of all the even integers from 2 to n, inclusive.  If p is the smallest prime factor of h(100) + 1, then p is

(A) between 2 and 10

(B) between 10 and 20

(C) between 20 and 30

(D) between 30 and 40

(E) greater than 40

  楼上的朋友，可否给出解题的思路？

第一個是20+16-1

因為自己被重複算了兩次