(1) When the numerator and denominator of the fraction are both 0, 2, 4, 6, 8. Not the simplest fraction, there must be a common factor of 2.

(2) When the numerator and denominator of a fraction are both 0 or 5, it is not the simplest fraction, and there must be a common factor of 5.

(3) When the sum of the numerator and denominator of a fraction is divisible by 3, it is not the simplest fraction, and there must be a common factor of 3.

(4) When the numerator and denominator are not above, it depends on whether there is a common factor of 7, 1 1, 13, 17, 19.

Method of restoration:

1, numerator denominator decomposition factor;

2. Find the common factor of numerator and denominator;

3. Exclude non-1 common factor.

If the greatest common divisor of numerator and denominator can be seen quickly, it is easier to divide by their greatest common divisor directly.