smc

Let the sum of the 10 numbers be x. The mean is then $\frac{x}{10}$. Then, since you're adding 20 to each number, the new sum of the numbers is x+200, since there are 10 numbers. Then, the new mean is $\frac{x+200}{10}$, which simplifies to $\frac{x}{10}+20$, which is $\boxed{\text{is increased by 20}}$.