Number Series
1. The sum of first 25 natural numbers is
(a) 325
(b) 327
(c) 320
(d) 376
Answer: (a)
Explanation:
1+2+3+…..+25
Sum of the First n Natural numbers
The symbol indicates a summation to be carried out.
= 25×13
= 325
2. Find the sum of 79 consecutive even numbers
(a) 7570
(b) 3560
(c) 6320
(d) 5520
Answer: (c)
Explanation:
2+4+6+. . . +158 (?79th even number is 79×2=158)
= 2(1+2+3+. . . +79)
= 79 × 80 = 632
3. The sum of first 25 whole numbers is
(a) 300
(b) 325
(c) 375
(d) 385
Answer: (a)
Explanation:
Whole numbers are starting from zero.
So, the sum of numbers from 0 to 24 
4. 
(a) 9070
(b) 7090
(c) 8060
(d) 6080
Answer: (a)
Explanation:
Sum of the squares of the First n Natural numbers,
= 9455-385
= 9070
5. The sum 
is equal to: [SSC Grad. 2000]
(a) 2295
(b) 2425
(c) 2495
(d) 2925
Answer: (a)
Explanation:
Sum of the Cubes of the First n Natural Numbers
=3025-100
=2925
6. Find the next nearest number in A.P: 2, 2. 5, ( ). .
(a) 3
(b) 5
(c) 4
(d) 9
Answer: (a)
Explanation:
If a constant quantity is added to or subtracted from each term of a given A.P, we get another A.P.
Example: 17, 20, 23, … is an A.P. Add 5 to each term
The general form of the A.P is a, a+d, a+2d, a+3d, . . .
Let us denote the sum to n-terms as Sn,
Then Arithmetic progression=2, 2. 5, ( )
d=2. 5-2=0. 5
- p=a, (a+d), (a+2d), …
- p=2, 2+0. 5, 2+ (2(0. 5)), … = 2, 2. 5, 3
So, next term is 3.
7. Sum of the series 5+10+15+20+ . . . is 240. Number of terms of the series is,
(a) 17
(b) 16
(c) 15
(d) 14
Answer: (c)
Explanation:
So, total number of terms is 15.
8. How many terms are in 4, 8, 16,… 2097152
(a) 20
(b) 19
(c) 18
(d) 21
Answer: (a)
Explanation:
4, 8, 12 . . . 2097152 is G. P
r=8/4=2, a=4
n-1 = 19; n=20
9. If n is odd, then sum of n odd numbers is 625 and sum of n even numbers is 600 then find number of natural numbers
(a) 44
(b) 48
(c) 49
(d) None of these
Answer: (c)
Explanation:
If n is odd then number of odd numbers
Sum of odd numbers
n =25
Sum of even number=n (n+1) = 600
n=24, 25
(n+1) is odd number
Therefore total number of odd number=odd number + even number
= 24 + 25 = 49
10. The sum of four numbers is 64. If you add 3 to the first number, 3 is subtracted from the second number, the third is multiplied by 3 and the fourth is divided by three, then all the results are equal. What is the difference between the largest and the smallest of the original numbers? [Bank 2000]
(a) 32
(b) 27
(c) 21
(d) cannot be determined
Answer: (b)
Explanation:
Let the number be a, b, c and d
Substituting these values in eq1
