In the classical theory of fuzzy sets, it has not been discussed how exactly to construct a fuzzy number in any particular case. In this article, we shall discuss how a trapezoidal fuzzy number appears in describing data represented in terms of intervals. In doing so we shall use the operation of superimposition of sets. Prices of perishable goods such as fish can be used to construct such fuzzy numbers. As a case study, we have collected data on the minimum and maximum daily prices of a particular variety of fish in a market of Guwahati, and we have shown that the data can be described by a trapezoidal fuzzy number.