LCM of denominators i.e 4 and 5 is 20. We try to make the denominaters equal to 20  3/4*5/5=15/20  and  4/5*4/4=16/20  when the denominators are same then the fractions can be compared easilythe fraction with greater numerator is greater and vice versa..so 4/5 is greater than 3/4