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Consecutive Numbers

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by anshumishra » Sat Dec 25, 2010 11:47 am
N:Dure wrote:From below, what's Not the sum of consecutive numbers starting from 1?

a) 10
b) 15
c) 30
d) 45


Any easier way than summing to find this out?
Sum = n*(n+1)/2
a): n(n+1) = 20 = 4*5
b): n(n+1) = 30 = 5*6
c). n(n+1) = 60 =
d). n(n+1) = 90 = 9*10

Hence , C.
Thanks
Anshu

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by Anurag@Gurome » Sat Dec 25, 2010 7:57 pm
N:Dure wrote:From below, what's Not the sum of consecutive numbers starting from 1?

a) 10
b) 15
c) 30
d) 45
Sum of consecutive n numbers starting from 1 = n(n + 1)/2.
=> 2*Sum = n(n + 1)
=> 2*Sum = Product of two consecutive integers

Let's check which one of the given options doesn't satisfy the above condition:
  • 1. Sum = 10 => 2*Sum = 20 = 4*5 => Possible
    2. Sum = 15 => 2*Sum = 30 = 5*6 => Possible
    3. Sum = 30 => 2*Sum = 60 = No such product! => Not possible
    4. Sum = 45 => 2*Sum = 90 = 9*10 => Possible
The correct answer is C.
Anurag Mairal, Ph.D., MBA
GMAT Expert, Admissions and Career Guidance
Gurome, Inc.
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