This review examines reasons for Malaysian pupils' underperformance in solving mathematics problems that demand numerical estimation and mental computation. Their underdeveloped number sense appears to be the major reason. According to the Ministry of Education, the national mathematics curriculum is unlikely to account for this underachievement. By contrast, this article argues that the curriculum largely focusing on written computation skills, is likely to account for the underperformance, given that the development of number sense has yet to become the focus of attention. This review further suggests teaching and learning activities that may enable the Malaysian pupils, in year one of primary schools, to conceive part-whole relations of numbers and thus develop number sense. The earlier they understand the part-whole relationship, the stronger they may develop number sense and thus the better can be their performance.