I like to work backwards to see if the common difference is the same.
So I take 16 and work to the left.
So I say 16 - 21 which is -5.
Then I do 21 - 26 which is -5.
Finally, 26 - 31 which is -5.
So what I am doing here is just taking the number that is on the right and subtracting it by the number that appears to the left of it.
Notice that I always get -5 and that has to be because in order for this to have a common difference, the result must always be the same if you are working with the method I used above.
So, our common difference is -5.
