Q. 164.1( 7 Votes )

Find the middle term of the sequence formed by all three-digit numbers which leave a remainder 5 when divided by 7. Also, find the sum of all numbers on both sides of the middle term separately.

Answer :

The numbers which leave 3 as remainder when divided by 7 will be in the form 7q + 5, for any integer q.


Hence, three digit numbers which leave 5 as remainder when divided by 7 are:


103, 110, 117, …, 999


Clearly above sequence is an AP, with


The first term, a = 103


Common difference, d = a2 – a1
= 110 – 103
= 7


Let the number of terms be ‘n’.


We know, nth term of an AP is


an = a + (n – 1)d


we obtain last term = 999


an = 999


103 + (n – 1)7 = 194


103 + (n – 1)7 = 999


7(n – 1) = 896


n – 1 = 128


n = 129


Number of terms = 129
Middle term will be:


middle term is 65th term.
Middle term is :
103 + 64(7) = 103 + 448
= 551

We know, sum of ‘n’ terms of an AP is



Now, sum of terms left to the middle term (64 terms), Sleft = S64



S64 = 32(206 + 63(7))

     = 32(206 + 441)
     = 32(647)
    = 20704

Now, the sum of terms right to the middle term(last 64 terms)


As 64th term is 551.
now 65th term will be 558.

= 32(1116 + 63(7))
= 32(1116 + 441)
= 32(1557)
= 49824

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