Question 1030071
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Here's a way to do it without algebra,
just basic math:

The LCM of 4 hours and 5 hours is 20 hours.

Crew A can paint 5 rooms in 20 hours and 
Crew B can paint 4 rooms in 20 hours.
So working together for 20 hours, they can paint 9 rooms 
So working together, they can 1 room in 1/9 that much time 
1/9th of 20 hours is 20/9 hours or 2 2/9th hours.

Edwin</pre></b>