Question

find the sum of all number between 250 and 1000 which are exactly divisible by 3.

Answer 1

The numbers between 250 and 1000 which are divisible by 3 are 252, 255, 258, ......,999

This is an A.P. whose first term a = 252, Common difference, d = 3 and Last term l=999

We Know that

l=a+(n-1)d

999=252+(n-1)3

999 - 252 = 3(n - 1)

n=250

We know that

S=n/2(a+l)

=250/2(252+999)

=125*1251

=156375

Hence, the required sum is 156375.(Ans)

Answer 2

See the attachment  for the answer