The simple-math approach:
The 20-ohm resistor is five times the 4, so its voltage drop will be (5 x 24) 120v.
E = I x R, so you need the current. 24 + 120 = 144, the circuit voltage, 4 + 20 = 24, the resistance.
I = E/R = 144 / 24 = 6a. E = I x R = 6 x 20 = 120v. P = E x I = 120 x 6 = 720w.