Luke and Dan's total debt is $72. If Luke's debt is three times Dan's debt, what is Dan's debt?

Luke and Dan's total debt is $72. If Luke's debt is three times Dan's debt, what is Dan's debt?

Let Dan's debt be d.
Let Luke's debt be l.

We're given two equations:

1. d + l = 72
2. l = 3d

Substitute equation (2) for l into equation (1):
d + 3d = 72

Solve for d in the equation d + 3d = 72

Step 1: Group the d terms on the left hand side:
(1 + 3)d = 4d

Step 2: Form modified equation
4d = + 72

Step 3: Divide each side of the equation by 4
4d/4 = 72/4
d = 18