The number 25a where a is a digit is divisible by 6. What is the largest possible value of a?
A number is divisible by 6 if it is both divisible by 2 and divisible by 3.
First, a number if divisible by 2 if it is even. Now, for 25a to be even, a must be even. Since a is even, it could be one of the following numbers: 0, 2, 4, 6 or 8.
Second, a number is divisible by 3 if the sum of its digit is divisible by 3. So, we substitute the possible numbers above, add the digits, and see if they are divisible by 3.
For a = 0