CAT 1998 — QA Question 1

In one day, A would do $\frac{1}{3}$ of the job, B would do $\frac{1}{4}$ of the job and C would do $\frac{1}{6}$ of the job.
Hence, if all three of them work simultaneously, in one day they would do $\left(\frac{1}{3}+\frac{1}{4}+\frac{1}{6}\right)=\frac{3}{4}$ of the job.
Hence, to complete the entire job together they would take $\frac{4}{3}$ days.
Shortcut:
A can complete the job in 3 days. So A, B and C combined will take less days than A alone to finish the job. So straightway option (b).