The same toy speed maps a later echo to a larger depth. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
A later echo has a larger one-way time
Round-trip time 8 s gives half-time 4 s.
8/2=4
The later reflector is deeper
The same speed gives depth 16 m, deeper than the 12 m echo.