Question 1124934
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When the faster machine works together with the slower machine, it is the same as if 4 slower machines work simultaneously.


So, then we know that 4 slower machine do the job in 6 hours, working together.


It means that one slower machine will complete the job in 4*6 = 24 hours.


Since the faster machine works three times faster, it needs only 24/3 = 8 hours.
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This problem can be solved mentally without using any equations, based only on your common sense.