The possible total costs are $54, $72 and $90
Here, we want to get the final possible costs
The ratio of the costs are; book to comics are 5 to 3
Hence, for every 5 books, 3 comics are bought
Thus, we have it that;
The cost 5(3) + 3(1) = 15 + 3 = 18
So, the possible total costs are multiples of 18
The multiples are 54, 72 and 90
