For this case we have the following variables: 
 x: number of months 
 f (x): number of caps sold 
 The function that models the problem is given by:
 

 By the time the store sells 64 caps we have: 
 

 From here, we clear the number of months: 
 

 Answer: 
 the store sells 64 caps in 3 monts 
 A. month 3