1) missing number is 7 (answer #2)
Each circles' numbers add up to 20. (the first one is 1 + 9 + 10, the second is 5 + 4 + 11.)
20 minus 10 minus 3 is 7, so that's the missing number.
2) missing number is 3 (answer #4)
This one's similar to the previous question. Each square's numbers have a sum of 10, this time. (First square is 2 + 7 + 1 + 0, second is 4 + 3 + 1 + 2.)
10 minus 6 minus 1 minus 0 is the same as 10 minus 7. This equals 3- so there's the number that's missing!
Hopefully this helps!