Echo time changes the out-and-back path before depth is halved. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Echo time stretches the round trip before depth is halved
The sound speed stays fixed; the scan changes only the echo time.
d=2ct
Three echo times give three exact depths
Each row recomputes path first, then halves it into depth.
c444t2610path82440d41220
The middle row is the six-second echo
The displayed depth cites the same checked row as the scan table.