A and B can together complete a task in 20 hours. After 5 hours A leaves, and B takes 45 hours to finish the remaining task. How many hours would A have taken to complete the entire task if he worked alone from the beginning?
A30 hours ✓ Correct
B40 hours
C48 hours
D60 hours
Correct answer: (A) 30 hours
Explanation
A alone would take 30 hours.
In 5 hours together they do 5/20 = 1/4 of the task.
B does the remaining 3/4 in 45 hours, so B's rate = 1/60.
A's rate = 1/20 - 1/60 = 2/60 = 1/30.
So A alone needs 30 hours.
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