explain please how we got the z forkula Examples

Example $9-210$ Three Fluids

Origin: Quiz $1801$

Determine the elevations $z$ in the tubes B and C if the pressure gage A reads $2$ kPa $.$ The specific gravity of gasoline and glycerin are $0\mathrm{.}7$ and $1\mathrm{.}25.$

Solution

The pressure at the bottom of the tank is

$p_b=2+1\mathrm{.}5(0\mathrm{.}7)9\mathrm{.}81+1(1\mathrm{.}25)9\mathrm{.}81=24\mathrm{.}6$kPa

The equivalent height of glycerin in tube $C$ is

$z_c=\frac{p_b}{1\mathrm{.}25{}_w}=2m$

Similarly, the height in tube B is obtained from

$p_b=1(1\mathrm{.}25)9\mathrm{.}81+(z_b-1)0\mathrm{.}7(9\mathrm{.}81)=24\mathrm{.}6$kPa

Hence, the height is $z_b=2\mathrm{.}8m.$