StatsFiddle's fraction arithmetic calculator allows fractions to be added, subtracted, multiplied and divided.
For addition or subtraction of fractions, it is important to find the Lowest Common Multiple of both Denominators. Once the lcm has been determined, scale up the numerators and denominators of both fractions to get a common denominator. Once this is done, the numerators can then simply be added or subtracted from eachother.
For multiplication, convert both fractions to improper fractions (if not already done), and then simply multiply numerator by numerator, and denominator by denominator.
For division, convert both fractions to improper fractions (if not already done), and then simply multiply the left numerator by right denominator (as the result numerator), and left denominator by right numerator (as the result denominator).
As a final step for all types of arithmetic, remember to check the result for Reduction as well as extract any whole numbers if the result is an improper fraction.