A number is divisible by 9 if the sum of its digits is greater than or equal to 9. 621 (6+2+1=9), for instance, is divisible by 9. Simply move the decimal point one place to the left to find the 10% ...

You probably haven’t had to do longhand math in years, but you do mental math every day. Or maybe you google math problems ten times a day, because you’ve forgotten how to do any math beyond your ...