The research examined the calculation methods used by pupils in Grades 3-6 when they were presented with problems that could be worked out efficiently and flexibly by applying number sense. The study was conducted with a convenience sample of 179 pupils between the ages 7 years and 10 months to 12 years and 10 months. in mainstream education in Israel, who attended schools belonging to different sectors and situated in different areas of the country with varied socioeconomic profiles. The test included addition, subtraction, multiplication, and division problems that pupils were asked to solve mentally, in writing and by identifying correctly and incorrectly solved problem. Some of the problems presented pupils with opportunities to apply number sense. As expected, the research findings showed significant differences in calculation accuracy between pupils with and without learning difficulties, especially in multiplication and division tasks. Still, the performance of pupils with difficulties in the accuracy variable was above average, and there was high variance within this group. We found significant differences between pupils with and without difficulties in the calculation-speed variable in all tasks and in all calculation modalities. One of the implications is that pupils, and especially those with difficulties, should be afforded enough time to work out problems, and should be presented with tasks that would enable them to use number sense in order to retrieve prior knowledge and apply it.