The sum of the first item and the last item is multiplied by the number of items (that is, how many numbers are there in a * * * *) and then divided by 2.

1 1+ 12+ 13+ 14+ 15+ 16+…+60

= ( 1 1 + 60) × 50 ÷ 2 = 1775

4 1+42+43+44+45+…+ 100

= (4 1 + 100) × 60 ÷ 2 = 4230