In a city 20 percent of the population travels by car, 50 percent travels by bus and 10 percent travels by both car and bus. Then persons travelling by car or bus is

Given $n\left(C\right)=20,n\left(B\right)=50,n(C\cap B)=10$.

We know that 

             $n(C\cup B)=n\left(C\right)+n\left(B\right)-n(C\cap B)$

Substitute the appropriate values in the above equation

Hence, 

              $\begin{array}{rcl}n\left(C\cup B\right)& =& 20+50-10\\ & =& 60\end{array}$

Hence required number of persons is $\boxed{60\%}$