First, you have to order the numbers from least to greatest which would be: 8, 9, 12, 18, 20.
This makes it somewhat easier.
Mean: The mean can be found by adding all of the numbers and dividing that by the amount of numbers there are. 8+9+12+18+20=67. Then, 67/5=13.4. So 13.4 is the Mean.
Median: The median is the number found in the middle of the numbers. Since we ordered it, we can look and see that 12 is in the middle, so it's the median.
Range: The range I'd the difference between the greatest number and the smallest number in the group. Again, since it's ordered, we can see that the smallest number is 8 and the biggest number is 20. 20-8= 12, so the range is 12.