List all the pairs of integers with a product of 36. then find the pair whose sum is 20

This is an excellent practice for the solution of quadratic equations.

1*36=36  => (1,36)
2*18=36  => (2,18)
3*12=36  => (3,12)
4*9=36   => (4,9)
6*6=36   => (6,6)
9*4=36   => (9,4)
12*3=36  => (12,3)
18*2=36  => (18,2)
36*1=36  => (36,1)

We can see that the sum decreases until the two factors are close (or equal) and then increases again.

The pair of integers with a sum of 20 is therefore (2,18) or (18,2).