1
a) 2~20 is the same as 1~19, therefore the answer is 19
b) 7~16 is the same as 1~10, therefore the answer is 10
c) 4~13 is the same as 1~10, therefore the answer is 10

2 Let us divide all the numbers in the set by 3:
1,2,3...14
Therefore the answer is 14.

You can also use the handy-dandy formula.
If A is an arithmetic sequence, then

n=(A_n-A₁)/r+1

(Wow, the subscripts work!)