Numbers which are divisible by 3 = 3, 6, 9, ........ ,99
Numbers which are divisible by 5 = 5, 10, 15, ........ ,100
Numbers which are divisible by 3 and 5 = 15, 30, ........ ,90
This is an que of AP
for A.P,
Sum of n terms=n/2(2a+(n-1)d)
Let S1 = 3+6+9..+99 = 1683
S2 = 5+10+15+...+100 = 1050
S3 = 15+30+45....+90 = 315
sum of the numbers between 1 & 100 which are exactly divisible by 3 or 5 = S1 + S2 -S3 = 2418