Question

how do you add together all the number's up to 100, Quickly though.

Answer 1

5050
formula
sum of sequence of numbers=(n/2)(first number plus last number)
n is total amount of digits
so total = (100/2)(1+100)
=50(101)
=5050

Answer 2

To get the formula, you need to do what Gauss did as a child. He paired the numbers up, reading from both ends:

1+2+3+4+...+97+98+99+100
is the same as
(1+100) + (2+99) + (3+98) + (4+97) + ... + (50+51)

There are 100 numbers, so there are 50 pairs adding to 101

sum = 101*100/2