Answer:
261
Step-by-step explanation:
If the farmer used 9 boxes and each had the same number, we need to find the number that's divisible by 9.
There's a great fact - a number is divisible by 9 if its sum of digits is divisible by 9.
Let's use that fact on the answers first:
281 -> 2 + 8 + 1 -> 11 -> 1 + 1 -> 2; not divisible by 9
273 -> 2 + 7 + 3 -> 12 -> 1 + 2 -> 3; not divisible by 9
261 -> 2 + 6 + 1 -> 9; divisible by 9
Please note that we could just say that 11 is not divisible by 9, but I wanted to show that we can do it again - nothing stops us and it still works.
To know why this method works, note that a number ABC is equal to:
100 * A + 10 * B + C = 99 * A + 9 * B + (A + B + C) = 9*(11*A + B) + (A + B + C)
And we know that 9*(11*A + B) is divisible by 9 because it's literally 9 times something. So A+B+C is equal to ABC minus something divisible by 9, and that proves what we needed.
