A can do $\frac{1}{5}$of the work in 3 days and B can do$\frac{1}{4}$ of the work in 7.5 days. Working together, A and B can do $\frac{4}{5}$ of the same work in:

8 days

A = \(\frac{1}{5}\)W = 3 days = 15 days,

B = \(\frac{1}{4}\)W = 7.5 days = 30 days,

⇒ \(\frac{4}{5}\) of total work = \(\frac{4}{5}\) x 30 = 24 units.

⇒ Time taken by A + B to complete 24 units = \(\frac{24}{2 + 1}\) = \(\frac{24}{3}\) = 8 days.     ..(\(\frac{Work}{Efficiency}\) = Time)