You find two numbers knowing that one is three times each other and add up to 120

IDK!

To set up the equation it would be:

3x + x = 120

Then you solve for x

4x = 120

x = 30

Then finally you substitute x = 30 into the original equation:

3 (30) + (30) = 120

90 + 30 = 120

The two numbers that add up to 120 and one is three times the other are: 90 and 30