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The Question is as follows:

The pressure head in a gas main at a point 120 m above sea level is equivalent to 180 mm of water. Assuming that the densities of air and gas remain constant and equal to 1.202 kg m^-3 and 0.561 kg m^-3, respectively, what will be the pressure head in millimeters of water at sea level.

my attempt was to use the densities given i.e (0.561/1.202) and multiply it by 120m.

then using the larger number as the height to calculate the pressure in the air, thus using the smaller number as the height used for calculating the pressure from the gas.

so i end up with the following steps

0.561*56*981 - pgh (gas)

1.202*9.81*64 - pgh (air)

sum those two values up and use the equation for pressure head,

P/(pg) = (pgh (gas) + pgh (air)) / (1000 *9.81) = 108 mm

however the answer in the book is 103 mm.

Any help is welcome,

Cheers - Iclaudius