The best thing to do in order to find the common factors is to find the factors of each number,
A factor is a number that the larger numbers (36 and 40) can be divided by.
The factors of 36 are- 1 2 3 4 6 9 12 18 36
The factors of 40 are- 1 2 4 5 10 20 40
As you can see, the numbers 1 2 and 4 are factors of both numbers- therefore, this makes them the common factors.
Hope this helps :)