Marcia is 3 times as old as Kate. The difference between their ages is 12 years. Find the age of each.

Without using algebra,

Difference of 12 years = (3-1)= 2 times Kate's age.
So Kate is 6, and so Marcia is 18.

Using algebra:
Let K=Kate's age,
Marcia is 3K.
Difference = (3K-K)=12 years
2K=12 years
K=6 years.
Marcia = 3*K = 18 years.