(a) We can find the current flowing between the walls by using Ohm's law:
where
is the potential difference and 
is the resistance. Substituting these values, we get
(b) The total charge flowing between the walls is the product between the current and the time interval:
The problem says
, so the total charge is
The current consists of Na+ ions, each of them having a charge of
. To find the number of ions flowing, we can simply divide the total charge by the charge of a single ion:
where
(b) The total charge flowing between the walls is the product between the current and the time interval:
The problem says
The current consists of Na+ ions, each of them having a charge of