Question 517460
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We know it has to be less than 3 days since painter a can do it alone in three days.

x/3+x/5=1
Why 1?
One is the whole job.
In one day painter a does 1/3 of the job and painter b does 1/5 of the job.

In x days painter a does x/3 and painter b does x/5 of the job.

5x+3x=15
8x=15
x=15/8
x=1 7/8 days
Assuming a work day is 8 hours, it will take 15 hours together.