1, 4.3 and 4.30 have the same counting units. (Wrong) The counting unit of 4.3 is 0. 1, and the counting unit of 4.30 is 0.0 1.

2. The class size is fixed, and attendance and truancy are inversely proportional. (Wrong), attendance+absence = total number of people.

3. After omitting the mantissa of "ten thousand", an integer is about 200,000, and the maximum number is 19999. (Wrong) The maximum is 20 4999.

4. If A and B are two different prime numbers, then a/b must be simplest fraction. (correct)