Aptitude - Numbers - Discussion
Discussion Forum : Numbers - General Questions (Q.No. 83)
83.
The sum all even natural numbers between 1 and 31 is:
Answer: Option
Explanation:
Required sum = (2 + 4 + 6 + ... + 30)
This is an A.P. in which a = 2, d = (4 - 2) = 2 and l = 30.
Let the number of terms be n. Then,
tn = 30 
a + (n - 1)d = 30
2 + (n - 1) x 2 = 30
n - 1 = 14
n = 15
 Sn = |
n | (a + l) | = | 15 | x (2 + 30) = 240. |
| 2 | 2 |
Discussion:
9 comments Page 1 of 1.
Shefali said:
1 decade ago
Why l is taken as 30, why not n ?. Please reply.
Vaoirakkani said:
1 decade ago
30 is the last even number in above problem.31 is odd number.so we drop 31
Asha said:
1 decade ago
Please solve the problem in some other easy method. Am not getting?
Achyuta said:
1 decade ago
Here 1 to 31, 15 even number. So n(n+1) = 15*16 = 240.
(1)
Sahid said:
1 decade ago
Total even number is n=15.
= 15/2*(2+30) = 240.
= 15/2*(2+30) = 240.
Rajat Kumar Sahoo said:
9 years ago
Sum of 'n' even numbers = n(n+1).
So, here n = 15.
Hence 15 * 16 = 240.
So, here n = 15.
Hence 15 * 16 = 240.
Nahida said:
8 years ago
M (M-1).
Here M = 30 + 2/2 = 16
So 16 *15 = 240.
Here M = 30 + 2/2 = 16
So 16 *15 = 240.
Sandeep Tanwar said:
8 years ago
Because n= No of terms, which is (15) because from 1 to 31,(only 15 even numbers). Whereas on the other hand.
L= last term (from 1 to 31,30 is the last even No).
L= last term (from 1 to 31,30 is the last even No).
Destello said:
3 years ago
Simplest way to solve this sum is;
Formula is: x(x+1).
To find value of x : n/2.
"n is last number of series, in our case 30"
x=30/2 =>15.
Put 15 into formula.
x(x+1).
15(15+1) = 15*16=240.
Formula is: x(x+1).
To find value of x : n/2.
"n is last number of series, in our case 30"
x=30/2 =>15.
Put 15 into formula.
x(x+1).
15(15+1) = 15*16=240.
(4)
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