Bingo! Times tables and Arithmetic Progression

Noone ever explained Arithmetic Progression to me as a kid.
Now it's so easy!! I sure could have solved some of those problems they gave me in highschool... (in fact, all of them).

Say if someone asked you to add all the numbers from 1 to 100.

Every number can be put together in pairs, for example, 100 + 1, and 99 + 2. They all equal 101. So there are 50 pairs (half of 100 numbers), so the total adds up to 101x50, which is 5050. (5000 + 50).

Arithmetic progression

I'm going to create a arithmetic progression.
It goes like this:

2 6 10 14 18 22 26 30 34 38

So there are 10 numbers.

so there are 5 pairs.

The first pair adds to 40. So 5 x 40 = 200.

Rule: To find the sum of an arithmetic progression, you add the first and the last number of the progression, and then multiply by a half of the number of numbers in the progression.

So it works for odd numbers too.
I want to find the answer to this progression for 13 numbers. So there are 6 and a half pairs.

6.5 x 40 = 220.