Calculate the average of the numbers 3, 6, 12 and 21.
The average of the numbers 3, 6, 12, and 21 is calculated by adding up all the numbers and then dividing by the total count of numbers, which is 4 in this case. 

So, 

Average = (3+6+12+21) / 4 = 42 / 4 = 10.5

Hence, the average of the numbers 3, 6, 12, and 21 is 10.5.
Below is an instruction that describes a task, write a response that appropriately completes the request.

### Instruction:
Calculate the average of the numbers 3, 6, 12 and 21.

### Response:
The average of the numbers 3, 6, 12, and 21 is calculated by adding up all the numbers and then dividing by the total count of numbers, which is 4 in this case. 

So, 

Average = (3+6+12+21) / 4 = 42 / 4 = 10.5

Hence, the average of the numbers 3, 6, 12, and 21 is 10.5.