Here you have a DC (Direct Current) source that is passing the different variations of resistors. Ohms law is V/IR (Voltage (V) divided by current (I) times resistance (R)). You can find 1/3 so long as you have the other 2. For instance, if you're trying to find current: V/R=I. Or if you want voltage: IxR=V.
Top circuit: V=10, R=1000 ohms, so 10/1000=0.010A or 10mA.
Second circuit: I=0.020A, R=500 ohms. So 0.020 x 500=10V.
Third circuit: I=0.005A, V=5V. So 5/0.005=1000 ohms.
