Add sequences of numbers in the 70's

1. Choose a 2-digit number in the 70's. To add all the 70's
   from 70 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 7 (+ carry)   X X _

   Example:

1. If the 2-digit number in the 70's selected is 73:
   (70 + 71 + 72 + 73 + 72 + 71 + 70)
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. 7 x no. of terms: 7 x 7 = 49   4 9 _
5. So the sum of the sequence is 499.

   See the pattern?

1. If the 2-digit number in the 70's selected is 78:
   (70 + 71 + 72 + ... + 78 + 77 + ... 72 + 71 + 70)
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. 7 x no. of terms: 7 x 17 + 6 = 49 + 6 =
   119 + 6 = 125   1 2 5 _
5. So the sum of the sequence is 1254.