Maria can build a model train in 7 hours. Anne can build a model train in 5 hours. How long would it take them if they work together?
They would need 2 hours and 55 minutes.
The key to this problem is to figure out how much of the complete train the two girls can each build per hour, assuming of course that they work at a constant rate.
This will give you an idea about how much of the train they can build together per hour.
So, if Maria can build a complete train in 7 hours, then she needs 1 hour to complete
Likewise, Anne can build a complete train in 5 hours, so she needs 1 hour to complete
This means that , together, they can complete
To determine how many hours they need to complete the train, find a number that multiplied with
To express this in hours, use the fact that one hour has 60 minutes
The total time will thus be