Multiple choice

A alone can complete a task in 3 days. B alone can complete the same task in 7 days. If A and B work together for 2 days, what fraction of the task will be left?

  1. $\frac{2}{21}$
  2. $\frac{1}{7}$
  3. $\frac{4}{21}$
  4. $\frac{1}{21}$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

A's work per day: 1/3. B's work per day: 1/7. Combined work per day: 1/3 + 1/7 = 7/21 + 3/21 = 10/21. Work done in 2 days: 2 × (10/21) = 20/21. Work remaining: 1 - 20/21 = 1/21.

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