Short Cut to Figure Out Divisibility of 3

Okay, you know the rule - add up all digits and divide the total by 3, if it's divisible by 3, then the whole number is divisible by 3. Simple right? But what happens if you have a super large number like 94857385958382? Do you still want to add them up before you divide? What if I tell you I can do it in a few seconds instead of minutes? Here is the trick - if your goal is to divide the total by 3, why not cross out the digits (or two digits added together) that are already a multiple of 3. Here is how it works - 1. Method One: 9+8+5+7+3+8+.....+2 = Whatever!! Did you add up all 14 numbers? 2. Mrs. Yu's way - 1.) get rid of 9, 4+5, 8+7, 3, 9, 3; 2.) you are left with 8, 5, 5, 8, 8, 2; 8+5+5 = 18, and so are 8+8+2 = 18, 18 is a multiple of 3, cross these numbers off! 3.) you are left with 0! Yes, this number 94857385958382 is divisible by 3! While others are busy adding up the total of the digits, you spend a few seconds crossing out the numbers that are already a multiple of 3. The result? You are done in a few seconds, and less likely to make mistake! Now can you challenge yourself to see if the same strategy can work for 9? Have fun figuring it out!!