Cascade filters multiply stage responses, so two cutoff stages each taking one half of a squared budget leave one quarter overall. The 20 to 10 to 5 ledger is exact for this ideal ratio model; real filters have component tolerance, loading, finite op-amp bandwidth, phase effects, noise, and measured frequency-response uncertainty.
Cascaded squared ratios multiply stage by stage. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
The first cutoff stage halves the squared budget
Input budget 20 becomes 10 after one cutoff stage.
20⋅21=10
The second stage multiplies again
The second half response makes final budget 5, not ten plus another correction.