Source: http://en.wikipedia.org/wiki/Singapore_math

Singapore math (or Singapore maths in British English^[1]) is a teaching method based on the national math curriculum used for kindergarten through sixth grade in Singapore.^[2][3] It involves teaching students to learn and master fewer mathematical concepts at greater detail as well as having them learn these concepts using a three-step learning process.^[2][3] The three steps are concrete, pictorial, and abstract. In the concrete step, students engage in hands-on learning experiences using concrete objects such as chips, dice, or paper clips.^[4] This is followed by drawing pictorial representations of mathematical concepts. Students then solve mathematical problems in an abstract way by using numbers and symbols.^[5]

The development of Singapore math began in the 1980s when the country’s Ministry of Education developed its own mathematics textbooks that focused on problem solving and heuristic model drawing.^[3][6] Outside Singapore, these textbooks were adopted by several schools in the the United States (U.S.) and in other countries such as Canada, Israel, and the United Kingdom.^[7][1][8] Early adopters of these textbooks in the U.S. included parents interested in homeschooling as well as a limited number of schools.^[3] These textbooks became more popular since the release of scores from the Trends in International Mathematics and Science Study (TIMSS), which showed Singapore at the top of the world three times in fourth and eighth grade mathematics.^[9] U.S. editions of these textbooks have since been adopted by a large number of school districts as well as charter and private schools.^[3]

Read more at: http://en.wikipedia.org/wiki/Singapore_math

The bar model can be drawn as a comparison model to compare two bars of unequal lengths, which can then be used to solve a subtraction problem.