tell the greatest digit to make the number divisible by 3

53

The number will be 1.

As 53+1= 54, which is divisible by 3.

You can check this by taking the sum of both the digits on ones and tens place, if it comes out to be divisible by 3, then it can be divided by 3.

In 54, 5+4= 9, which can be divided by 3.