The total time is (1h=3600 s):
[tex]\Delta t=6.0 h=21600 s[/tex]
The current intensity I is the amount of charge Q that passes through a certain point in a time [tex]\Delta t[/tex]:
[tex]I= \frac{Q}{\Delta t} [/tex]
Since we know the current, [tex]I=3.5 mA=0.0035 A[/tex], we can find how much charge has been trasferred from one terminal to the other in 6 hours:
[tex]Q= I \Delta t=(0.0035 A)(21600 s)=75.6 C[/tex]
