Given,
Number of days by Rakesh to complete the project \(=10\) days
Number of days taken by Ravi to complete a project \(=15\) days
\(\because\) Rakesh takes 10 days to complete a project.
\(\therefore\) Work done by Rakesh in a day \(=\frac{1}{10}\)
\(\because\) Ravi takes 15 days to complete a project.
\(\therefore\) Work done by Ravi in a day \(=\frac{1}{15}\)
Let Anas takes \(X\) days to complete the project.
Work done by Rakesh in 4days + work done by Rakesh and Ravi in
2 days \(+\) work done by all three persons in \(\frac{16}{13}\) days \(=1\)
\(\Rightarrow \frac{4}{10}+2 \times\left(\frac{1}{10}+\frac{1}{15}\right)+\frac{16}{13} \times\left(\frac{1}{10}+\frac{1}{15}+\frac{1}{X}\right)=1\)
\(\Rightarrow \frac{2}{5}+2 \times\left(\frac{5}{30}\right)+\frac{16}{13} \times\left(\frac{5}{30}+\frac{1}{X}\right)=1\)
\(\Rightarrow \frac{1}{3}+\frac{16}{13} \times\left(\frac{1}{6}+\frac{1}{X}\right)=1-\frac{2}{5}\)
\(\Rightarrow \frac{16}{13} \times\left(\frac{1}{6}+\frac{1}{X}\right)=\frac{3}{5}\)
\(\Rightarrow \frac{16}{13} \times\left(\frac{1}{6}+\frac{1}{X}\right)=\frac{3}{5}-\frac{1}{3}\)
\(\Rightarrow \frac{1}{6}+\frac{1}{X}=\frac{13}{16} \times \frac{4}{15}\)
\(\Rightarrow \frac{1}{X}=\frac{13}{60}-\frac{1}{6}\)
\(\Rightarrow \frac{1}{X}=\frac{1}{20}\)
\(\Rightarrow X=20\)
\(\therefore\) Number of days taken by Anas to complete the project \(=20\) days.
Hence, the correct answer is 20.