Circuit Solution via Incremental Quantities

Electricity and Magnetism Level 4

In the circuit shown below, $r_1$ and $r_2$ are resistors such that $\frac{r_1}{r_2}=4$ . If we connect a lamp as shown, the current through resistor $r_1$ increases by $\SI{0.1}{\ampere}$ .

Find the current through the lamp (in amperes).