Despite the emphasis that children should have a robust sense of number and a thorough understanding of fraction (National Mathematics Advisory Panel, 2008), many students continue to struggle with these concepts. Booker Diagnostic Assessment Framework (Booker, 2011) can inform decision about teaching that improves students' learning outcomes. Focusing on decimal numbers, this paper reports how effective use of the framework and instructional materials can help three teachers challenge their students' misconceptions with mathematics, leading to improved learning outcomes.