Renamed prime.rkt to math.rkt.
Added factorial, nPr and nCr to math.rkt.
Added problem 15.
Fixed math.rkt provide line.
Implemented problem 15.
Implemented problem 16.
Added problem 17.
Implemented problem 17.
Added problem 18.
Wrote solution to problem18.
Wrote a better solution.
Implemented problem 18.
Added problem67.rkt
Implemented problem 67.