Learning or doing mathematics involves approximate quantification. The purpose of this research was to improve approximate number system acuity in primary school students with low achievement in mathematics. A quasi-experimental, pre-test, post-test design with a control group was utilized. Mathematics Achievement Test (MAT), Arithmetic Performance Test (APT), Number Line Estimation Test (NLE), and Raven Standard Progressive Matrices Test (RSPMT) have been conducted to identify the study groups. Initially, 302 students were surveyed with these tests. A total of 26 students scoring the lowest (bottom 25%) in all these tests, except RSPMT, have been included in the study. Students were randomly assigned to the experimental (13 students) and control groups (13 students). Experimental group played with Tablet-PC games designed to develop approximate number system, one of the components of number sense, in their free times in school. Control group, did not have any of these games but played nonmathematical games. Experimental group played with three games designed to develop number sense for two hours a week for a total of 6 hours. Analysis showed there was an increase in both of the estimation precision and mathematics achievement of the experimental group. The games played during the experimental process not only helped in teaching the spatial representation of magnitude but they also led to an improved mathematics achievement. The approximate number system sensitivity of experimental group continued to develop in retention period as measured by the 0-100 number line test. Despite an indication that number line estimation tasks have an impact on number sense and mathematics achievement, none of these results provided any evidence on being reflected on timed arithmetic performance. Activities targeting exact number system may be required for arithmetic performance. This hypothesis can be tested in future studies.