Walk down each column to one common level

A U-tube’s connecting liquid is at rest. The two interfaces between the connecting liquid and the columns above it are at the same horizontal level. Above the left interface sits 0.30 m of water (1000 kg/m³) with gas pressure pg on top. Above the right interface sits 0.50 m of oil (800 kg/m³) open to atmospheric pressure pa.

What is the gauge pressure pg − pa?

Organize the givens & assumptions

Water height = 0.30 m; density = 1000 kg/m³
Oil height = 0.50 m; density = 800 kg/m³
g = 10 m/s²

Each column is immiscible with the connecting liquid; both bottom interfaces are at equal height in the same continuous stationary connector. Neglect capillary pressure and gas density.

A $-1000\ \mathrm{Pa}$
B $7000\ \mathrm{Pa}$
C $2000\ \mathrm{Pa}$
D $1000\ \mathrm{Pa}$

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