The mean of 20 numbers is 20. If 2 is added to each of the first 10 numbers, find the mean of the new set of 20 numbers.

$\text{Mean} = \dfrac{\text{Sum of all observations}}{\text{Total number of observations}}$

∴ Sum of observations = Mean x Total number of observations

⇒ Sum of observation = 20 x 20 = 400.

Since 2 is added to each of the first 10 numbers, the total sum increases by 10 x 2 = 20.

New sum = 400 + 20 = 420

New Mean = $\dfrac{420}{20}$ = 21

Hence, the mean of the new set of 20 numbers is 21.