Description for Correct answer:
A's 1 day's work = \( \Large \frac{1}{21} \)

B's 1 day's work = \( \Large \frac{1}{28} \)

Therefore, Same money is sufficient to pay the wages of both for

= \( \Large \frac{1}{\frac{1}{21}+\frac{1}{28}} \)

= \( \Large \frac{21 \times 28}{21+28} = \frac{21 \times 28}{49} = 3 \times 4 \) = 12 days