XtraMath uses spaced repetition to help students strengthen their recall of basic arithmetic facts. This works sort of like smart flash cards — the program determines which facts are easy for the student, and focuses on the facts they are less fluent in. Students are presented with facts based on their current strengths and weaknesses, not preset groups of facts.
XtraMath is a great tool for practice, progress monitoring, and informal assessment, but it does not teach strategies. You should start by teaching or reviewing concepts, using concrete examples and manipulatives. Students should be able to solve problems without time pressure before practicing those operations in XtraMath.