Calculate the potential difference across each resistor in parallel. 12v dc supply, 10 ohms (R) 5 ohms(R)?

When resistors are in series, their resistance can be added directly.

When resistors are in parallel, they add as follows

R(effective)=\frac1{\frac1{R1} + \frac1{R2} }

R(effective) = \frac{R1 + R2}{R1 R2}

After you figure out the resistance, it's Ohm's law all the way. The potential drop across each resistor will be identical since they're in parallel.