Discussion

A four-digit number 3x15 is a multiple of 3. Then, the sum of all the possible values of x is :

Answer: D

The sum of the possible values of \(x\) is 18.

A number is a multiple of 3 when the sum of its digits is a multiple of 3. For 3x15:

\[ 3 + x + 1 + 5 = 9 + x \]

Since 9 is already a multiple of 3, \(x\) itself must be a multiple of 3. As a single digit, \(x\) can be 0, 3, 6 or 9, and

\[ 0 + 3 + 6 + 9 = 18 \]

Leaving out 9 gives 9, and leaving out 0 changes nothing, so 9, 6 and 12 all miss at least one valid digit.

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