geffa, youve been tricked. I learned this one while studying magicians tricks (an old hobby of mine).
Any 3 numbers multiplied by 3 numbers will reduce to a number divisible by 3 when you add the numbers up in the total PROVIDED that the first or second number is a product of 3. As you are using 2 numbers, he has a strong possibilty (ODDS ON) of you following the rule, and he stays silent when you dont.
eg 567 times 432= 244944 Now add 2+4+4+9+4+4=27, then 2+7+9. 9 is divisible by 3, so the answer is correct. Both 567 and 432 are products of 3.
853 times 270= 230310= 9 As 270 is a product of 3, the numbers in the total MUST add up to 3 or 6 or 9. Other totals like 18, just get broken down again, so 8 plus 1 = 9. so the rule holds again.
853 times 271=231163=16=7 so we know that either a) 853 or 271 are not divisible by 3, or b) IF one of them is divisible by 3, then the total must be incorrect.
I bet thats been bugging you for years. Did he ever tell you the totals BEFORE the guy with the calculator answered? There are a few speed geniuses (geneii?) out there that dont use tricks, but they are few and far between.
