Write down three fractions.
Calculate a number that all three denominators will fit into evenly. Find the lowest number that all three will fit into if possible. This is known as the least common denominator. For example, if you have 1/2, 4/5 and 7/15, the lowest number that all three will fit into is 30.
Multiply the numerators by the number of times that the original denominator fits into the common denominator. For example, in the fraction 1/2, the number 2 fits into the common denominator (30) 15 times. Multiply 1 times 15 to get 15. Therefore this fraction becomes 15/30. In the fraction 4/5, 5 fits into 30 six times. Multiply 4 times 6 to get 24, so that the new fraction is 24/30. Finally, in the case of 7/15, 15 fits into the common denominator twice, so you would multiply 7 times 2 to get 14/30.