Charge flowing thugh AB when the switch is open.

 

Equivalent capacitance = 10*5 / 15 = 10/3

.: Charge flowing through AB = CV = 10/3*50 = 500/3 C

 

When the key is closed  ;

 

no current at all would pass through the capacitor which is connected with it (the one just below the key).

.: we can say that the equivalent capacitance is 10

 

so charge flowing = 10*50 = 500 C

 

So the charge which flows through AB when the switch is closed is

 

=500 - 500/3

 

=333.3C

 

=333.3 * 10^-6 C

 

=3.3 * 10^-4 C