Answer:
780
Explanation:
So when you're adding consecutive numbers like this. 1+2+3...+39. You add 39 and 1. This is 40. 2+38 is 40. This is a pattern. Now, we do 40 multiplied by 39 and this will give you the number 1620. After this you would have to divide this product by 2. The answer will soon be 780.