this is sort of weird to write without any numbers, but it's not too hard. take your info out of the word problem:
fit fast = fee per class
stepping up = fee per class + monthly fee
now you just change all of that into variables, essentially:
F = cx
S = cx + m
so, fit fast is the cost of classes (c) times the number of classes taken (x). stepping up is the cost of classes times x, PLUS that additional monthly fee (m).
