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What is the units digit of the sum of all positive multiples of 6 smaller than 600?

 Aug 4, 2021
 #1
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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

 Aug 4, 2021

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