Consider a U-tube whose arms are open to the atmosphere. Now equal volumes of water and light

Question:

Consider a U-tube whose arms are open to the atmosphere. Now equal volumes of water and light oil (ρ = 49.3 lbm/ft³) are poured from different arms. A person blows from the oil side of the U-tube until the contact surface of the two fluids moves to the bottom of the U-tube, and thus the liquid levels in the two arms are the same. If the fluid height in each arm is 40 in, determine the gage pressure the person exerts on the oil by blowing.