Which of the following is not a multiple of 9?

To determine which number is not a multiple of 9, one effective method is to use the rule of divisibility for 9. This rule states that a number is a multiple of 9 if the sum of its digits is also a multiple of 9.

Let's apply this rule to each of the options:

• For the first number, 1188: The sum of the digits is 1 + 1 + 8 + 8 = 18, which is a multiple of 9.

• For the second number, 981: The sum of the digits is 9 + 8 + 1 = 18, which is also a multiple of 9.

• For the third number, 540: The sum of the digits is 5 + 4 + 0 = 9, which is a multiple of 9.

• For the last number, 408: The sum of the digits is 4 + 0 + 8 = 12, which is not a multiple of 9.

Thus, since the sum of the digits in 408 does not yield a total that is a multiple of 9, this confirms that 408 is not a multiple of 9.