Greg’s Golf Shop hires teenagers to collect lost golf balls from the city’s golf courses. The golf balls are then packaged in boxes of a dozen for reselling. This summer, the workers collected 2,244 golf balls. After repackaging, how many golf balls will be left over?
Explanation
Answer: They will all fit into the boxes, with no leftovers. This problem is asking you whether 2,244 is divisible by 12. It is, because 2,244 is divisible by both 3 and 4 (which is how you tell if a number is divisible by 12).
Key Takeaway: There are quick ways to tell if a large number is divisible by smaller ones.
A number is divisible by 2 if its last digit is even.
A number is divisible by 3 if, when you add all 3 digits together, that sum is divisible by 3.
(In the answer above, adding 2+2+4+4 = 12; 12 is divisible by 3, so 2,244 is as well.)
A number is divisible by 4 if the last 2 digits of the number is divisible by 4.
(In the answer above, 44 is divisible by 4, so 2,244 is divisible by 4.)
A number is divisible by 5 if the last digit is 5 or 0.
A number is divisible by 6 if it is even, and divisible by 3.
To determine if a number is divisible by 7, take the last digit, double it, then subtract it from the rest of the number. If the resulting number is divisible by 7, the original number is. For example, consider 132.
Take the last digit (2), double it (4), then subtract that from the rest of the number (13). 13-4 is 9. Nine is not divisible by 7, so 132 isn’t either.
A number is divisible by 8 if the number formed by its last 3 digits is divisible by 8.
A number is divisible by 9 if, when you add the digits together, that sum is divisible by 9.
