Sharon is 42 years old. her daughter is 12 years old. how many years ago was sharon 4 times older than her daughter?

To do this, we need to know how much older than her daughter Sharon. She is 30 years older, so we get
d = s - 30
We want to figure out when d = s - 30, and 4d = s, so here's what we do:
Since 4d = s, we can substitute 4d for all occurences of s
d = 4d - 30
And then we can solve for d
-3d=-30
d=10
And then we plug that number into our original equation
10 = s-30
40 = s
Sharon was 40 years old when she was 4 times her daughter's age. Since she is now 42, we know she was 4 times her daughter's age 2 years ago