The all-letters item type — 'which of the following expresses r in terms of A, P, and t' — with no arithmetic, no graph, and no numbers for Desmos to plot, which is exactly why it is a permanent resident of hard Module 2: the difficulty migrated to where the calculator cannot follow. The doctrine: treat every letter except your target as if it were a number; A=P(1+rt) is the same problem as 20=5(1+2r).