Aptitude - Numbers - Discussion

Discussion :: Numbers - General Questions (Q.No.101)

101. 

The sum of even numbers between 1 and 31 is:

[A]. 6
[B]. 28
[C]. 240
[D]. 512

Answer: Option C

Explanation:

Let Sn = (2 + 4 + 6 + ... + 30). This is an A.P. in which a = 2, d = 2 and l = 30

Let the number of terms be n. Then,

a + (n - 1)d = 30

     2 + (n - 1) x 2 = 30

    n = 15.

Sn = n (a + l) = 15 x (2 + 30) = (15 x 16) = 240.
2 2


Mumu said: (Jan 4, 2019)  
Can anyone explain the formula clearly?

Suriya Prakash said: (Feb 14, 2019)  
How did n=15 comes? Here n=14 only.

Shabbir said: (May 12, 2019)  
Here,
2+(n-1)*2 = 30,
(n-1)*2=30-2 = 28,
(n-1)=28/2 = 14,
n=14+1 = 15.

Bahun said: (Aug 1, 2019)  
n(n+1).
15 * 16,
= 240ans.

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