The equation "a/b = c/d" represents a proportion, where the ratio of two numbers, a and b, is equal to the ratio of two other numbers, c and d. This means that if you divide a by b, you will get the same result as dividing c by d. Proportions are often used in mathematics to compare quantities and solve problems involving ratios. In practical terms, if you know three of the four values in the proportion, you can find the fourth. For example, if a is 2, b is 4, and c is 3, you can find d by setting up the equation: 2/4 = 3/d. This helps in various fields, including science, finance, and engineering, where understanding relationships between quantities is essential.