A stone fell from the top of a cliff into the ocean
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Manny can assemble a computer in 50 minutes. That means, in a minute Manny does 
job.
Lucy can assemble a computer in 45 minutes. That means, in a minute Lucy does
job.
If they are working together, in a minute they will do:
In a minute they will do
job
To finish the job they will do it in
minutes or about 24 minutes.
Lucy can assemble a computer in 45 minutes. That means, in a minute Lucy does
If they are working together, in a minute they will do:
In a minute they will do
To finish the job they will do it in