What is the sum of all natural numbers from 51 to 100, both included?

Explanation

1. 01

   Sum from 1 to $100 = \frac{100 \times 101}{2} = 5{,}050$.

2. 02

   Sum from 1 to $50 = \frac{50 \times 51}{2} = 1{,}275$.

3. 03

   Sum from 51 to $100 = 5{,}050 - 1{,}275 = 3{,}775$.