Answer :

Aazaran
Alright, take it step by step...in month one, they had 5 members.
if the number doubles each month, then the second month they will have a total of ten members, right? in the third they will have twice the number before, or 2 times 10, 20. if you double 20, you have the number of people in the fourth month, giving you a grand total of 40.
Since you know that the number of club members doubled every month , you would multiply how much members in one month by 2 
1 month= 5  members multiply by 2
2 month= 10 members multiply by 2
3 month = 20 members multiply by 2 
4 month= 40 members

So, for each moth you multiply it by 2.
After 4 months there will be 40 members