Let distance from Q to R equal x. Then distance from P to Q is 4x. So we have something like this:
P-------------Q-----R
4x x
Distance all the way from P to R is 4x + x = 5x, and this must be equal to 120. So set up the equation:

Divide both sides by 5 to isolate x:

This is the length of QR.
Check to be sure:
P-------------Q-----R
4(24) + 24
96 + 24 = 120
Works!