How to add 1 through 100 quickly
If you want to add up all the numbers 1 through anything, like 1 through 100, or 1 through 23, there’s actually an easy way to do it. You might think you’d have to grab your calculator and manually add up 1+2+3+4…, all the way until you get to your ending number. But there’s a much faster way to do this addition problem; it’s by turning it into a multiplication problem.
If you’re adding the numbers 1-n, the quick trick is to use the formula (n/2)(n+1). So if for example you’re trying to add up all the numbers 1-64, then you know right away that the sum of all those numbers is (64/2)(64+1)=32*65. As you can see, the tedious, time-consuming addition problem becomes a simple multiplication problem.