Management Aptitude Test 2015 Paper

(c) (A + B)'s 1 day’s work = $\frac{1}{10}$
(B + C)’s 1 day's work = $\frac{1}{15}$
(A + C)’s 1 day's work = $\frac{1}{12}$
Now, (A + B + C)’s 1 day’s work
= $\frac{1}{2}\left(\frac{1}{10}+\frac{1}{15}+\frac{1}{12}\right)$
= $\frac{1}{2}\left(\frac{6+4+5}{60}\right)$
= $\frac{1}{2}\left(\frac{15}{60}\right)$ = $\frac{1}{2}\left(\frac{1}{4}\right)=\frac{1}{8}$
Hence, A, B and C together can finish the work in 8 days