Question 807168
12/a+12/b=1,
16/b+16/c=1,
d/a+d/b+d/c=1
a = 16,   b = 48,   c = 24,   d = 8
all three do the job in 8 hours.
check
12/16+12/48=1
3/4+1/4=1

16/48+16/24=1
1/3+2/3=1

8/16+8/48+8/24=1
1/2+1/6+1/3=1
ok