Answer:
D. 18.
Explanation:
3 5 9 13 16 20
To find the average we add these up and divide by 6:
= 66 / 6
= 11.
Now we want to increase the average by 1 that is to 12 by adding one more number.
So our new total has to be 12* 7 = 84.
So our new number is 84 - 66 = 18.