Context: Human development
Zara's teacher notes how Zara solves simple addition problems, such as 3 + 6, at several points during her first year at primary school, when she is five to six years old. Early in the year she usually counts out both numbers on her fingers and then counts all of them from one, taking a long time over each problem. Later in the year she sometimes starts from the larger number and counts on. By the end of the year she answers many problems quickly and correctly without counting, but on harder problems, or when she is tired, she sometimes goes back to counting on her fingers.
With reference to this scenario, how one continuous model of development could account for the changes in Zara's addition.