Summary of the article:
Van Galen, F. & D. v. Eerde. (2013). Solving Problems with the Percentage Bar. IndoMS. J.M.E, 4(1), 1-8.
In an observation toward 14 Grade VII students of a achool in the Netherlands, surprising result (only 4 students could solve the problem correctly) was performed by the students when dealing with the following problem:
Trying to solve the problem, the students directly did calculations. None of them tried to make visual representation to see the relations between the given numbers. As the consequence, they often went wrong.
Actually, the students have tried several ways to find the answer like working with equal proportion or playing with the numbers in the question.
The former one was supposed to bring them to the expected result. However, they were stuck since they have to work with unfriendly proportion (15 out of 100) which led them to confused. While the latter one was really out of concept. The students might think that the problem must involve simpler calculation. So, they just tried to divide 600 over 15 and conclude that the answer should be 40.