Pressure P at a depth H is given by:-
P=P° + Hdg(Since pressure increases as we go down from a surface of a fluid and P° is pressure at topmost level ,d is density and g is accln. due to gravity)
P = 4x10^4 Pa
4x10^4 = 10^4 + (h)(10^3)(10)
=> we get
h= 3 metres...