Question 28935
we are given two condition i.e 
1. we need to calculate h such that it is a whole num..
2. fees should be more than $15 but less than $20

also we are given the equation ... F = 5h+4

If we see the second condition which says that fees should be more than $15 but less than $20,,,so we are left with only few options i.e fees or F is to be equal to either 16 or 17 or 18 or 19 ....

Moreover we are given in first condition that we need to calculate h such that it is a whole num..so 

F or fees should be such that after subtracting 4 from fees or F, the variable we get which we get must be exactly divisible by 5 ...

Hence only 19 can justify all the above condition,,,

F = 5h+4

when f = 19...we get 

19 = 5h + 4

19 - 4 = 5h 

15 = 5h

15/5 = h 

or h = 3 hours...

hope this will help you 

Please feel free to revert back for any further queries.