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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.