Question 1110216: Given the sequence 3;6;9;....;60.
a)Determine the sum of all the natural numbers from 1 to 60 which are not multiples of 3.
Answer by math_helper(2461)   (Show Source):
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The key to solve this is to recall that Sum(1,2,…, N) = N*(N+1)/2
Sum(3,6,9,…,60) = 3*Sum(1,2,3,…,20) = 3*(20*21/2) = 630
Sum(1,2,3,…,60) = 61*60/2 = 1830
Sum of natural numbers which are not multiples of 3: 1830-630 = 
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