Two resistors of 15 and 30 W are connected in parallel. If the combination is connected in series with a 9.0-V battery and a 20-W resistor, what is the current through the 15-W resistor?

Kirchhoff's junction rule: At any junction point, the sum of all the currents entering the junction must equal the sum of all the currents leaving the junction.
Kirchhoff's loop rule: The sum of the changes in potential around any closed path of a circuit must be zero.