Erin is 8 years younger than twice Paula's age. The sum of their ages is 40. How old is Erin

You do the sum (40) subtracted by 8, the answer from that divided by 2 since Erin's two times younger.

40-8=32

32:2=16

Then if you want to check it you do the opposite and get the same sum (40) so 16*2=32 and 32+8=40