Murad can do a piece of work in $33$ hours. If he is joined by Joel who is $100\%$ more efficient, in what time will they together finish the work?

Let the efficiency of Murad be $1$ unit/hour.Since Murad takes $33$ hours to complete the work:
$$\text{Total Work}=\text{Efficiency}\times \text{Time}=1\times 33=33\text{ units}$$
Joel is $100\%$ more efficient than Murad:
$$\text{Efficiency of Joel}=1+\left(\frac{100}{100}\times 1\right)=2\text{ units/hour}$$
When they work together, their combined efficiency is:
$$\text{Combined Efficiency}=\text{Efficiency of Murad}+\text{Efficiency of Joel}=1+2=3\text{ units/hour}$$
The time taken by both of them to finish the work together is:
$$\text{Time}=\frac{\text{Total Work}}{\text{Combined Efficiency}}=\frac{33}{3}=11\text{ hours}$$