Aptitude - Numbers - Discussion
Discussion Forum : Numbers - General Questions (Q.No. 101)
101.
The sum of even numbers between 1 and 31 is:
Answer: Option
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 |
Discussion:
7 comments Page 1 of 1.
Yes yes said:
6 months ago
We knoe that, sum of even numbers = n (n+1),
Here, n is the number (amount) of even numbers.
There are 15 even numbers in between 1 to 31.
Substiting in the formula,
n(n+1) = 15(15+1)= 15(16)= 240.
Here, n is the number (amount) of even numbers.
There are 15 even numbers in between 1 to 31.
Substiting in the formula,
n(n+1) = 15(15+1)= 15(16)= 240.
(1)
Mukesh said:
4 years ago
Form the formula, here a=2,Tn =30,d=2
Tn=a+[n-1]*d
30 =2 +[n-1]*2.
On solving this equation we got n=15.
Now put this value in the sum of the series formula;
Sn=n/2[2a+(n-1)*d],
We get 240.
Tn=a+[n-1]*d
30 =2 +[n-1]*2.
On solving this equation we got n=15.
Now put this value in the sum of the series formula;
Sn=n/2[2a+(n-1)*d],
We get 240.
(2)
Rupa said:
5 years ago
Please explain the answer in detail way.
(2)
Bahun said:
6 years ago
n(n+1).
15 * 16,
= 240ans.
15 * 16,
= 240ans.
(1)
Shabbir said:
6 years ago
Here,
2+(n-1)*2 = 30,
(n-1)*2=30-2 = 28,
(n-1)=28/2 = 14,
n=14+1 = 15.
2+(n-1)*2 = 30,
(n-1)*2=30-2 = 28,
(n-1)=28/2 = 14,
n=14+1 = 15.
(2)
Suriya prakash said:
7 years ago
How did n=15 comes? Here n=14 only.
(1)
Mumu said:
7 years ago
Can anyone explain the formula clearly?
(1)
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