Addition is a/b + c/d = (a * d + b * c) / (b * d)
Subtraction is a/b - c/d = (a * d - b * c) / (b * d)
Multiplication is a/b * c/d = (a * c) / (b * d)
Division (a/b) / (c/d) = (a * d) / (b * c)