UPSC NDA Math Model Paper 7

The numbers between 300 and 500 which are divisible by 7, are 301, 308, 315,....,497.
This is an A.P with first of term = 301.
Therefore from the nth term of A.P = a + (n - 1) d,we have,
497 = 301 + (n -1)7
⇒196 = (n - 1)7
⇒ n - 1 = 196/7 = 28
⇒ Number of terms = n = 28 + 1 = 29
Now, sum =

n

2

(a+l)
=

29

2

(301+497)
=

29×798

2

=29×399
=11571
Hence, option (4) is correct.