236 cans
1st trip
start with 60 cans, travel 8m, dump 44 cans, return
2nd trip
start with 60 cans, travel 8m, get 8 cans, travel 12m, dump 36 cans, turn around,
travel 12m, get 8 cans, return
3rd trip
start with 60 cans, travel 8m, get 8 cans, travel 12m, get 12 cans, travel 20m, dump 20 cans, turn around,
travel 20m, get 12 cans, travel 12m, get 8 cans, return
last trip
start with 56 cans, travel 8m, get 12 cans, travel 12m, get 12 cans, travel 20m, get 20 cans,
travel 60m (for a total of 100m)
number of cans consumed = 236