Add sequences of numbers in the 20's

1. Choose a 2-digit number in the 20's. To add all the 20's
   from 20 up through this number and down from it:
2. Square the 2nd digit of the number
   (keep the carry)  _ _ X
3. The number of terms is 2 x the 2nd digit + 1.
4. Multiply the number of terms by 2 (+ carry).   X X _

   Example:

1. If the 2-digit number in the 20's selected is 23:
   (20 + 21 + 22 + 23 + 22 + 21 + 20)
2. Square the 2nd digit of the number: 3 x 3 = 9   _ _ 9
3. No. of terms = 2 x 2nd digit + 1: 2 x 3 + 1 = 7
4. 2 x no. of terms: 2 x 7 = 14   1 4 _
5. So the sum of the sequence is 149.

   See the pattern?

1. If the 2-digit number in the 20's selected is 28:
   (20 + 21 + 22 ... + 28 + 27 + ... 22 + 21 + 20)
2. Square the 2nd digit of the number: 8 x 8 = 64
   (keep carry 6)   _ _ 4
3. No. of terms = 2 x 2nd digit + 1: 2 x 8 + 1 = 17
4. 2 x no. of terms (+ carry): 2 x 17 + 6 = 40   4 0 _
5. So the sum of the sequence is 404.