A and B can do a job in 7 days. A is 13/4 times as efficient as B. The same job can be done by A alone in ?

Answer: Option B

(A's 1 day's work) : (B's 1 day's work)
$$\begin{array}{rl}& =\frac{7}{4}:1\\ & =7:4\end{array}$$
Let A's and B's 1 day's work be 7x and 4x respectively
Then,
$$\begin{array}{rl}& 7x+4x=\frac{1}{7}\\ & \Rightarrow 11x=\frac{1}{7}\\ & \Rightarrow x=\frac{1}{77}\\ & \therefore \text{A’s 1 day’s work}\\ & =\left(\frac{1}{77}\times 7\right)\\ & =\frac{1}{11}\end{array}$$
Hence, A alone can do the job in 11 days.