In the circuit shown ${\mathrm{C}}_1={\mathrm{C}}_2=2\mathrm{\mu F}$ . Capacitor C₁ is charged to 50 V and C₂ is charged to 20 V. After charging, they are connected as shown. When S₁, S₂ and S₃ are closed

When switches are open, charges on the capacitor are $100\mathrm{\mu C}$ and  $40\mathrm{\mu C}$ shown. When switches are closed, capacitors become parallel; charges are shown on two capacitors. 

Since the left plate was having a charge of 100 $\mathrm{\mu C}$initially, and the final charge on the left most plate is 30$\mathrm{\mu C}$, We can conclude that a charge of 70$\mathrm{\mu C}$ has flown through switch S₁.