Rational number density has been investigated through open-ended question tasks and multiple-choice tasks or by asking to interpolate a number between two numbers. However, students’ responses to these three types of tasks were not directly compared. The objective is to look for relationships between the three types of tasks in order to identify differences regarding students’ density understanding depending on the type of knowledge elicited. Participants were 791 primary and secondary school students. Results show that most of the students believed that rational numbers are discrete. Differences between the modes of representation are also found. Finally, interpolating a number between two pseudo-consecutive ones is neither a necessary nor a sufficient condition for students to answer that there are infinitely many intermediate numbers.