Summary of the article:
Van Galen, F. & D. v. Eerde. (2013). Solving Problems with the Percentage Bar. IndoMS. J.M.E, 4(1), 18.
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.
What should teachers do?
A number of manipulatives would be alternatives to teach the concept of percentage to students. However, percentage bar was chosen since it clearly displays the relationship of the total and its parts.
What is the percentage bar and how does it work?
Simply, here is the picture of percentage bar:
This tool, according to van den Heuvel (2003); van Galen et.al (2008); Rianasari et al. (2012), has several advantages, such as:

Students could make their own representation on the relationship between the known and the questioned variable.

Students could observe their thinking process, visualize their performance, propose their plans, and reflect their works.

Students could derive working with the unit percent naturally.
Does it really work?
Teaching experiment has been conducted to see how students adopt with this tool. The learning begins by introducing the calculation via 10% (of course with easier numbers). Afterward, calculating via 1% was also given to deal with more complicated numbers.
During the experiment, the students seemed to very quickly adopt with the manipulative. They could also recognize the purpose of the tool and even solve the given problems correctly.
The overall explanation shows how percentage bar could successfully deal with the students’ problems on that material. Such tool is effective and avoid students from memorizing steps or work under ‘trick’ solution.
TO NOTE: It is important to first introduce students to work with easier forms like 50%, 25%, or 10%, unless the students would probably infer an incorrect conclusion.
For any problems to solve, understanding is needed and the percentage bar helps to build understanding.
I suggest to anyone who reads this post to find the original article. A very obvious explanation to the use of this manipulative is there.. π