Working 5 hours a day, A can Complete a work in 8 days and working 6 hours a day, B can complete the same work in 10 days. Working 8 hours a day, they can jointly complete the work in:

Answer: Option A

1^st Method:
Working 5 hours a day, A can complete the work in 8 days i.e.
= 5 × 8 = 40 hours
Working 6 hours a day, B can complete the work in 10 days i.e.
= 6 × 10 = 60 hours
(A + B)'s 1 hour's work,
$$\begin{array}{rl}& =\frac{1}{40}+\frac{1}{60}\\ & =\frac{3+2}{120}\\ & =\frac{5}{120}\\ & =\frac{1}{24}\end{array}$$
Hence, A and B can complete the work in 24 hours i.e. they require 3 days to complete the work.

2^nd Method:
% 1 hour's work of A = $$\frac{100}{40}$$ = 2.5%
% 1 hour's work of B = $$\frac{100}{60}$$ = 1.66%
(A + B) one hour's % work,
= (2.5 + 1.66) = 4.16%
Time to complete the work,
= $$\frac{100}{4.16}$$ = 24 hours
Then, $$\frac{24}{8}$$ = 3 days
They need 3 days, working 8 hours a day to complete the work.