The aim of this research was to look at the strategies used by children when doing mental addition problems of the varying levels of complexity. An authentic arithmetic task was designed for use in a school environment. The central aim was to study developing fluency in mental arithmetic as achieved through recruiting various strategies into solving more complex problems than those studied by existing research. The nature of mental addition strategies was inferred from children's solution times when doing sequences of sums. Three studies were carried out on 7-11 year-old children from two local schools.