resistor problem

Equivalent resistance in the parallel arrangement
= (10+20)//30 ohms = 30//30 ohms = 30x30/(30+30) ohms = 15 ohms

hence voltage across the parallel arrangement
= 12 x 15/(15+15) v = 6 v

voltage across 20-ohm resistor = 6 x 20/(10+20) v = 4 v

power dissipated in 20-ohms resistor
= 4^2/20 w = 0.8 w