For the third image, 8/20, isn't there an error on the last row of the problem? I am using the XOR method you described (add images and remove overlapping areas). Anyway, I believe the solution is the fifth option.

For the first image, 4/20, answer is 75. For each row, add the two numbers, and then reverse the digits. If the addition results in a number over 100, add the 1 to the last digit, then reverse the resulting two digit number.

For the second 9/20, answer is 62? Multiply 2 numbers outside of circle, subtract the multiplication of the two remaining numbers, and you have the number in the middle of the circle.

Thanks for all the answers, esp. 11/20! I forgot that equation from high school math.
I have an alternate solution for 9/20, the shapes problem.
6 3 5 4 4
\ \ / \ / /
\ \ / \ / /
\ \ / \ / /
\ \/ \ / /
\ /\ \ / /
\ / \ / \ /
\ / \ / \ /
-1 \ +1 -1 ? ----> +1 (so 4+1 = 5)
So are there multiple solutions to these problems?

I am unable to find similar problems or solutions online, so I thought I'd ask here...I'd really appreciate it if you all could help me learn how to solve these types of problems, so please include the correct reasoning process and solution, if you don't mind.
Thanks in advance!