Question

how many multiples of 9 lie between 70 and 700 ? find the sum of these multiples of 9.
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Answer 1

Here series will we = 306, 315, 324 ,333 ,......693. So, we have the first term a =306 and d=9 and last term l= 693

693= 306 +(n-1)9

77= 34+(n-1)

n=77-34+1

= 44

sum=n/2 (a+1)

=44/2(306+693)

=22(999)

= 21978 answer