This study explored how a teaching experiment, which taught double counting with a constructive task assignment according to an ongoing assessment, helped students with mathematics difficulties develop their multiplicative reasoning skills. The participants were two fifth graders with math learning disabilities and one at risk. A micro-genetic approach with a single-participant design was used. Investigators coded and analyzed four strategies the children used. Results showed that unitary counting was dominant during the baseline sessions. During the teaching experiment, a great increase in the use of double counting and an obvious decrease in the use of unitary counting was observed. The participants' improvement in problem-solving accuracy on the posttest validated their strategic development. Implications are discussed in the context of employing strategic changes as a measure for assessing students' academic achievement and interventions for helping children adopt advanced strategies. (Contains 3 tables and 2 figures.)