SSC CHSL 20 Dec 2015 Shift 2 Paper

Let the total work to be done = L.C.M. (2,10) = 10 units
Let efficiencies of A, B and C be and units/day respectively.
A, B and C together can finish of a work (i.e. 5 units) in 2 days, ⇒ $\left(\mathit{a}+\mathit{b}+\mathit{c}\right)=\frac{5}{2}=2.5$ units/day ------(i)
Similarly, $\left(\mathit{b}+\mathit{c}\right)=\frac{3}{2}=1.5$ units/day ------(ii)
Subtracting equation (ii) from (i), we get :
⇒ $\mathit{a}=2.5-1.5=1$ unit/day
∴Time taken by A alone to complete the work = $\frac{10}{1}$ = 10 days