The other tutor's answer is incorrect.
Two painters were working on a house. Painter A can complete the given job in 8
hours
Painter A's rate is 1 job per 8 hours or 1/8 job per hour
but painter B takes 10 hours.
Painter B's rate is 1 job per 10 hours or 1/10 job per hour
They worked together for the first four hours,
Their combined rate is 1/8 + 1/10 = 5/40 + 4/40 = 9/40 job per hour.
In those 4 hours at that rate they did 4(9/40) or 9/10 of the job.
So 1/10 of the job remained to be done.
but painter A left and told painter B to finish the job.
How long would it take for painter B to finish the job?
Let the answer be x hours. Then rate times time = the remaining 1/10
B's rate is 1/10 jobs per hour, so the equation is rate times time = 1/10.
(1/10)(x) = 1/10
x = 1 hour.
Edwin