Competence in fractions is important in achieving advanced mathematics such as algebra and calculus. To foster students' understanding of fractions, intervention studies have found number lines to be an effective representational tool. Yet, it is unclear whether or not number lines are equally effective regardless of the ways in which they are implemented. The present study compared two ways in which number lines were used to teach fourth graders fraction magnitudes. One was an iPad digital game and the other was a paper-and-pencil workbook. The students in the digital-game condition played the game on their own whereas those in the workbook condition received one-on-one tutoring. Taking part in just four lessons, both groups improved their fraction knowledge compared to the control group. The current findings confirm that number lines are an effective representational tool. The results further suggest that number lines -- when implemented as a digital game or in a workbook -- are on the average equally effective in improving students' fraction knowledge. Implications for classroom instruction are discussed.