Larry Barrett
Another way to get to 3 is to look at the sum of the digits in each cluster:
4+5+2+0=11; 6+7+4+2=19; difference between these is 8.
So sum of the last cluster should be 27; 8+9+7+3=27.
I do not see any other result other than 3 and 5.
Happy New Year all.