Look at the denominators of the fractions you wish to compare. If the denominators are identical, for example 2/5=3/5, then the fraction with the largest numerator (3/5) is the largest fraction. If the denominators are unequal, you'll need to cross multiply to determine which fraction is the largest.
Cross multiply the fractions to get two cross-product values. The first value is determined by multiplying the first numerator by the second denominator. The second value is determined by multiplying the second numerator by the first denominator. In the example 2/3=4/5, the first value is calculated by the equation 2x5=10, so the first value is 10. The second value is determined using 4x3=12, so the second value is 12.
Compare the cross-product values -- if they are equal, the fractions are equal; if the first value is higher, the first fraction is larger; if the second value is higher, the second fraction is larger. In the example above, the second fraction (4/5) is the larger fraction because 12 is greater than 10.