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!