A and B started working on a project and A can complete the project in 36 days. A worked for 20 days and B completed the remaining work in 8 days. How many days would B have taken to complete the entire project all by herself?

Correct option is A

​$$\text{A can complete the work in } 36 \text{ days} \Rightarrow \text{A's 1 day work} = \frac{1}{36} \\[6pt]\text{A worked for 20 days:} \\\Rightarrow \text{Work done by A} = 20 \times \frac{1}{36} = \frac{20}{36} = \frac{5}{9} \\[6pt]\text{Remaining work} = 1 - \frac{5}{9} = \frac{4}{9} \\[6pt]\text{B completed } \frac{4}{9} \text{ work in 8 days} \\\Rightarrow \text{B's 1 day work} = \frac{4}{9} \div 8 = \frac{4}{72} = \frac{1}{18} \\[6pt]\Rightarrow \text{B alone can complete the whole work in } \boxed{18 \text{ days}}.$$​​