What is the equivalent fraction of 18 over 42?

When reducing fractions (finding an equivalent fraction) you have to determine a common divisor of both the numerator (top number) and the denominator (bottom number). For simplicity's sake 18 and 42 are both divisible by 2 so they can be reduced to 9 over 21.

When each factor is multiplied by 2 again the fraction is still equal to 18/42. However, in this instance you can see that 9 and 21 still have a common factor and can still be reduced. Consider the number 3 this time. After dividing again the most simplified equivalent factor is 3 over 7.

(The most simplified form of a fraction is often what most math professors look for)