Convert all the mixed fractions to improper fractions. If the mixed fraction is A B/C, the conversion is to AC/B. For example 3 1/2 = 7/2 because 2 x 3 + 1 = 7. Also, 3 3/4 = 15/4 because 3 x 4 + 3 = 15.
Write all the fractions with the same denominator. You cannot add 3/4 and 4/5 because neither nas the same denominator. An easy way to make two fractions have the same denominator is to multiply both parts of the first fraction by the denominator of the second fraction and vice versa. This does not change the values of the fractions because multiplying the numerator and denominator of a fraction by the same number is like multiplying by N/N or multiplying by 1. After doing this procedure, the denominators of both fractions will be the same -- they will both be the products of the two original denominators. For example, 3/4 + 4/5 = (3 x 5) / (4 x 5) + (4 x 4) / (5 x 4) = 15/20 + 16/20 = 31/20.
Add more than two fractions by adding two at a time until there is only one fraction left. For example, 2/3 + 3/4 + 4/5 = (2/3 + 3/4) + 4/5 = ((2 x 4) / (3 x 4) + (3 x 3) / (4 x 3)) + 4/5 = (8/12 + 9/12) + 4/5 = 17/12 + 4/5 = (17 x 5) / (12 x 5) + (4 x 12) / (5 x 12) = 85/60 + 48/60 = 133/60. For some purposes, this would be the final answer. If you need to convert it to a mixed fraction, divide 60 into 133 and express the remainder as a fraction with 60 in the denominator -- 133/60 = 2 13/60.