Komal can do a work in $51$ days. Bimal is $50\%$ more efficient than Komal in doing the work. In how many days will Bimal do that work?

Let the efficiency of Komal be $2\text{ units/day}$.
Since Bimal is $50\%$ more efficient than Komal, Bimal's efficiency is:

$$\text{Bimal’s efficiency}=2+\left(50\%\text{ of }2\right)=2+1=3\text{ units/day}$$

The ratio of efficiency of Komal to Bimal is $2:3$.

Total Work = $\text{Efficiency of Komal}\times \text{Time taken by Komal}$

$$\text{Total Work}=2\times 51=102\text{ units}$$

Time taken by Bimal to complete the work:

$$\text{Time}=\frac{\text{Total Work}}{\text{Bimal’s efficiency}}=\frac{102}{3}=34\text{ days}$$