There is a "formula" for this. But notice when we add the first number, 1, and the last number, 100, we get 101. And addnig the next-to-the-last number, 99, and the second number, 2, we also get 101. And, continuing in this manner, we have 50 pairs of numbers that each add to 101. Therefore, 50 * 101 = 5050.
You can think of it like this
0, 1, 2, ......................49
100,99,98,........................51
--------------------------------add
100+100+100..................100 , The only missing number is 50
So answer must be 100*50+50=5050
There is a "formula" for this. But notice when we add the first number, 1, and the last number, 100, we get 101. And addnig the next-to-the-last number, 99, and the second number, 2, we also get 101. And, continuing in this manner, we have 50 pairs of numbers that each add to 101. Therefore, 50 * 101 = 5050.