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b=hours it takes boy to do the job.
b+8=hours it takes Alex to do the job.
1/b=part of the job Boy can do in 1 hr.
1/(b+8)=part of the job Alex can do in 1 hr.
Together they can complete a job in 15/2 hours so they can do 2/15 of the job in one hour.
1/b + 1/(b+8) = 2/15
15(b+8)+15b=2b(b+8) multiply each side by 15b(b+8) to eliminate fractions.
b=12 hrs it takes Boy working alone.
b+8=20 hrs it takes Alex working alone.