When the key is off there should be no current in the circuit.
Our ammeter still measures some current, and this is called hysteresis error. This means that this particular ammeter is going to add some amount of current to any current measured. In order to compensate for this error, we simply we deduct it from all measurements.
Each mark on the ammeter is worth:
[tex]m=\frac{100mA}{20}=5mA[/tex]
This means that our error is:
[tex]e_r=3\cdot m=15mA[/tex]
When the circuit is closed we measure:
[tex]I_{er}=200mA+17\cdot m=285mA[/tex]
This is current with the error, real current in the circuit is:
[tex]I=I_{er}-e_r=285-15=270mA[/tex]
