ACT question that do not know how to solve

The painter can paint one house in 20 hours, so his rate of working is 1/20 house per hour. His assistant would take 4 times as longâ€”80 hours.

His assistantâ€™s rate is 1/80 house per hour.

Their combined rate is 1/20+1/80=4/80+1/80=5/80=1/16 house per hour.

1/16 house per hour = 16 hours for one house, so it takes them 16 hours working together.

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