in question A2 part (ii) why P2 is considered to be zero
P2 is given as atmospheric and P1 is given as a gauge pressure of 80kN/m^2.
The gauge is assumed to measure the pressure against atmospheric ie it could be a Bourdon tube type gauge.
Which means if atmospheric is taken as 1 then the inlet pressure would be 80 + 1 kN/M^2 absolute. Then, one would subtract 1 from the 81 to get the pressure difference...