The intention of the present study is to know how students' performance on number sense based on the components of number sense and also the sub-components in it. The participants of the study are 80 six graders (12-13 year-old) from three different schools that represent the city, rural, and small town areas. The data were collected using the test. The data analysis showed that the elementary school students' performance on number sense was still weak on the component of understanding the meaning and concept of numbers. This could be seen in 23.53% respondent. The highest average was 49.75% in understanding the meaning and effect of operation. Nevertheless, the students' responses indicated that most of those were more dominated by the written algorithms in solving problems. This was also happened on the component of applying knowledge and number sense and operation in computational situation. The students found some difficulties in understanding the meaning and concept of numbers, especially on the domain of fraction and decimal. There were some obstacles the students had, such as misconception about the density of fraction and decimals, about the concept of the part of fraction, and some errors when doing the computation because they paid more attention to the rules and algorithms they understood.

The intention of the present study is to know how the pupils can learn to make a group of ten to understand the idea of unitizing. The pupils were given a contextual problem "Counting the Beads" in order to promote their understanding about the idea of unitizing. The process of designing the problem was based on the 5 tenets of Indonesian Realistic Mathematics Education (IRME). The methods of this study was a design research. The researcher designed the Hypothetical Learning Trajectory (HLT) before conducting the lesson in the classroom. The result of this study showed that the pupils learned to make a group of any number then moved to make a group of twenty before using the group of ten as a strategy in finding the amount of the beads. The pupils set a structure of objects with a help of grouping to ten strategy to understand the idea of unitizing.