What is the units digit of the sum of all positive multiples of 6 smaller than 600?
+191 We find a pattern: 6, 12, 18, 24, 30, 36, 42 and the rest of it.
It repeats every five numbers. 6, 36, 66, 96 or 12, 42, 72, 102.
We add 6+2+8+4+0 and multiply that by 20, but now we also include 606, so we subtract 6 from that.
20*20 is 400, minus 6 is 394.
So the units digit is 4
