Multiple choice

A can paint a house in 21 days and B in 42 days. If they work together for 7 days, what fraction of work is left?

  1. $\frac{3}{2}$
  2. $\frac{1}{4}$
  3. $\frac{2}{3}$
  4. $\frac{1}{2}$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

A's work rate is 1/21 per day, B's is 1/42 per day. Combined rate = 1/21 + 1/42 = 3/42 = 1/14 per day. In 7 days, they complete 7 × 1/14 = 1/2 of the work. Therefore, work left = 1 - 1/2 = 1/2. Option C (2/3) would mean they only completed 1/3, which is incorrect.

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