to much working out but just as a summary section of working out
3*15=45
300+45=$345(1st year)
345+300=645
6.45*15=96.75
645+96.75=$741.75(2nd year)
keep on doing that until you beat 2 in list and use common sense to realise that (d) is too big, and (b, a) is too small so (c) is the correct amount. (this is quicker than actually doing all 20 years)
