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What is the sum of the units digits of all the multiples of 3 between 0 and 50? 

 
 Jan 6, 2019
 #1
avatar+812 
+5

From 1 to 30, there are 10 multiples of 3: 3, 6, 9, 12, 15, 18, 21, 24, 27, and 30. The sum of the units digits is 3+6+9+2+5+8+1+4+7+0 = 45. 

Since the digits go up to 50, not 60, we stop the next list of addents up to 18. 3+6+9+2+5+8 = 33. 33+45 = \(\boxed{78}\).

 

You could also just use brute force and add them all up.

 
 #2
avatar+3715 
+2

You can add them up by grouping keenly: 3+6+9+2+5+8+1+4+7+0+3+6+9+2+5+8, pair 3 with the 7, 6 with the 4, 9 with the 1 and so on. Finally, you'll get \(\boxed{78}.\)

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 Jan 6, 2019

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