The current I is defined as the quantity of charge Q that flows in the tube in the unit of time t:
[tex]I = \frac{Q}{t} [/tex]
In our problem, we have [tex]I=7\cdot 10^{-5} A[/tex] and the time is t=5.0 s. Therefore we can find the charge that hits the screen during this time:
[tex]Q=It =7\cdot 10^{-5} A \cdot 5.0 s=3.5X10^{-4} C[/tex]