Level
1

Find the last digit of $1+2+3......+100$ .

1
8
4
0

**
This section requires Javascript.
**

You are seeing this because something didn't load right. We suggest you, (a) try
refreshing the page, (b) enabling javascript if it is disabled on your browser and,
finally, (c)
loading the
non-javascript version of this page
. We're sorry about the hassle.

This is a sum of an arithmetic progression .

$\begin{aligned} S & = 1+2+3+...+100 \\ & = \sum_{k=1}^n k & \small \color{#3D99F6}{n=100} \\ & = \frac {n(n+1)}2 \\ & = \frac {100(100+1)}2 \\ & = 5050 \end{aligned}$

The last digit of the sum is $\boxed{0}$ .