I did it by finding their LCM (lowest common multiple) 24 hours. Now it’s easier to understand that three of them peel 9 sacks(first-4, second-3, third-2) each 24 hours. If we know that to peel 9 sacks they need 24 hours, how many hours do they need to peel 87 sacks?
P.S. I guess you could also do it without “the 24 hours” thing, just by finding how many(what portion, to be precise) sacks do each of them peel in one hour. First-1/6, second-1/8, third-1/12 and add them up to find total # of sacks peeled in one hour. It’s 9/24=0.375 and I hope the rest is evident for you