The sum of two numbers is 48. One number is 3 times as large as the other. What are the numbers?

A+b=48, b=3a, using b from the second equation in the first equation gives you:

a+3a=48 combine like terms on the left side

4a=48 divide both sides by 4

a=12, since b=3a

b=36

So the numbers are 12 and 36