A normal interface reflects by an exact impedance ratio. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Impedance contrast fixes the reflection ratio
The interface from 2 rayl to 6 rayl gives numerator 4 and denominator 8.
R=84=1/2
Pressure amplitude follows the signed ratio
Incident pressure 4 pascals times ratio 1 over 2 gives reflected pressure 2 pascals.