For the first one, with Bob and James, it takes Bob 1 roof for every 16 hours per roof ( 1/10) James can finish 1 every 16 hours, ( 1/16) 1/10 + 1/16=1/t 8/80 + 5/80=1t 13/80=1t now you would cross multiply 13(t)=1(80) 13t=80 t=6.15 It would take them 6.15 hours to finish!
