imc

Since the average of the first $20$ numbers is $30$, their sum is $20\cdot 30=600$. Since the average of $30$ other numbers is $20$, their sum is $30\cdot 20=600$. So the sum of all $50$ numbers is $600+600=1200$ Therefore, the average of all $50$ numbers is $\frac{1200}{50}=\boxed{24}$