A checked target gate accepts 12 m and 14 m but rejects 16 m. 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 target gate includes the endpoint
Target 12 m with tolerance 2 m accepts depths 12 m and 14 m.
14−12=2
A deeper echo fails the target gate
Depth 16 m differs from the target by 4 m, so it is rejected.