- How do you know if a number is divisible by 3?: sum of digits divisible by 3
- how do you know if a number is divisbile by 9: sum of digits are divisible by 9
- how do you know if a number is divisble by four: last two digits are div by 4
- how do you know if a number is divisible by six: even and divisble by three
- what is the work formula: (x*y)/(x+y)