SOLUTION: 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.

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): You can put this solution on YOUR website!

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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