Changing speed at fixed round-trip time complements the echo-time scan. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Two metres per second gives six metres
Hold round trip time at 6 s, so half-time is 3 s. Speed 2 m/s gives depth 6 m.
2⋅(6/2)=6
Four metres per second gives twelve metres
The same echo time with speed 4 m/s gives depth 12 m.
4⋅(6/2)=12
Six metres per second gives eighteen metres
Speed 6 m/s gives depth 18 m. This is still a toy one-dimensional echo ledger.