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6 Machines, each working at a constant rate, together can complete the job in 12 days. How many additional machines, working at the same rate are needed to complete the job in 8 days.
Same here. I get 3.
We know they work at a constant rate... say each machine take T days
6 machines .. it takes 12 days
so, 6/T = 1/12
Lets say N machines take 8 days
so, N/T = 1/8
Substitue value of T in either equation
you get N = 9
So 3 more machines. _________________