968 362 396 392 139 963 359

To determine if the number above is divisible by 3, only fi ve of the 21 digits actually need to be added together. Find this 'reduced' digit sum.

If this number is well examined there are six 9s, four 6s, six 3s ... If add up these numbers together it'll always give a multiple of three because the numbers itself (9,6,3) are multiple of 3. So the numbers you need to add are 8,2 (twice),1 and 5 ... 8+2+2+1+5=18 ... Hence the number is divisible by 3 ...