It looks like there might be a missing number in your question. You asked for the average of "12, , and 18," but one number is missing. If you meant to find the average of just 12 and 18, here's how you do it:
Average=12+182=302=15\text{Average}=\frac{12+18}{2}=\frac{30}{2}=15Average=212+18=230=15
If there's a third number you want to include, please provide it, and I can calculate the average for you!
