[plt-scheme] HTDP exercise 4.4.3

I solved the problem by making an auxiliary for each slice of the amount that 
returns the payback for that slice. The main payback function only adds the 
results from these functions.

It returns the right values but I wonder if it is the expected way to solve 
this problem because the cond is spread in the auxiliary functions and the 
main function does not contain a cond which seems to be the opposite of the 
other functions shown in the book so far.

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neige27