jimrate=1/18boat per day
work=rate times time
1boat=(jimrate+harryrate) times 12
jimrate=1/18 boat per day
1boat=(1/18+harryrate) times 12
1boat=12/18+12harryrate
1boat=2/3+12harryrate
minus 2/3
1/3=12harryrate
times 1/12 both sides
1/36=harryrate
so harry needs 36 days
C
