From the figure it is seen that the voltage source of 10 V is in parallel to the resistance R1 and R2. The current due to the applied voltage through R1 and R2 is not related as the two are connected in parallel. To determine the current flowing through R1 use the formula V = I*R where V is the voltage applied across the resistor with resistance R and I is the current that flows through it as a result.
10 = 5*I
=> I = 2 A
A current of 2 mA flows through the resistor R1.