Bernoulli's Equation and pressure differences

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1. Nov 5, 2016

OnAHyperbola

1. The problem statement, all variables and given/known data
Problem in attached image

2. Relevant equations

$$P_1+\frac{\rho v_1^2}{2}=P_2+\frac{\rho v_2^2}{2}$$

3. The attempt at a solution
I understand everything in the solution except why $$P_A-P_B=h(\rho_{Hg}-\rho)g$$ Why do we have to subtract the density of water from that of mercury instead of just taking the density of mercury??

Any help is appreciated.

2. Nov 5, 2016

TSny

Consider starting at point B and moving down through the U-tube all the way around until you come up the left side of the U-tube to point A. Keep track of all the pressure changes.