You've got the idea. Parts (a) and (b) are essentially the same - to get the lower bound, you use the smaller value (so for 0-1, you use 50, for 1-2, you use 70, etc.). To get the upper bound, you use the larger value (so for 0-1, you use 70, for 1-2, you use 97, etc.). In (a) you sum the 5 intervals from 0 to 5, and in (b) you sum the 8 intervals from 0 to 8.
To do (c), you use your upper and lower bound for how much has leaked from 0 to 8, and calculate how much time it takes to empty the tanker at a constant 720 gal/h in both cases.