Because multiples of 3 can be odd or even.

For example, Wang Ning ate for 7 days, so the total is 2 1, which is an odd number.

If you eat for 8 days, it is 24, which is an even number.

So what Xiaohong said makes no sense.

2. Xiaohong was born on 19981October 9.

Because the first number from left to right is neither prime nor composite, it is 1.

The number in the second place is the smallest multiple of 9, which is 9.

The third and fourth digits are the largest even number less than 100, that is, 98.

Xiaohong was born in a double-digit month, which is a multiple of 2 and 5 and is 10.

Xiaohong's birth date is the largest odd number in 10, which is 9.

3. Because the circumference of this rectangle is equal to twice the sum of its length and width, and the number of centimeters of its length and width is a natural number, the sum of its length and width must be a natural number, and twice the natural number must be an even number. So the circumference of this rectangle must be even.