The sum of sally and beth's age is 60. six years ago, sally was 3 times as old as beth. what are their present ages?

It's quite simple. Look:

Lets let S- be the age of Sally and B- can be the age of Beth

S+B=60
S-6=3(B-6)

S-6=3B-18
S=3B-18+6
S=3B-12

3B-12+B=60
4B=60+12
4B=72
B=72/4
B=18

B=60-18
B=42

The ages are (B)18 and (S)42

Hope this helps!! :)