The involvement of working memory in children's exact and approximate mental addition

► Children's mental calculation with carrying increases WM demands. ► Horizontal addition problems are more impaired than vertical ones by verbal WM load. ► Vertical problems are more impaired than horizontal ones by visuospatial WM load. ► The specific involvement of WM components are also related to the problem complexity. ► The effect of presentation format is stronger in approximate than in exact operations.