A can do a certain work in 15 days, while B can do the same work in 21 days. If they work together, then in how many days will the same work be completed ?

$7\frac{3}{4}$ $9\frac{3}{4}$ $6\frac{3}{4}$ $8\frac{3}{4}$

$8\frac{3}{4}$

A = 15 days, B = 21 days, ⇒ time taken to complete the work together = \(\frac{105}{7 + 5}\) = \(\frac{105}{12}\) = \( { 8}_{4 }^{ 3} \) days.    ..(\(\frac{Work}{Efficiency}\) = Time)