let A = set..div by 3
B : set..div by 5
A^B : div by both 3 and 5
A = 3, 9, 15.....495, sum1 = n/2(first+last)(WE CAN'T TAKE 6,12...THESE ARE EVEN)
B= 5, 15, 25.....495 calculate sum2
A^B = 15,45,..... 495calculate sum3
use AUB (div by 3 or 5 )= sum1 + sum2 - sum3