What is the depth x of the oil?

[looi76]

Homework Statement

The bore hole of depth $$2000m$$ contains both oil and water as shown. The pressure at the bottom is $$17.5MPa$$. The density of the oil is $$830kgm^-3$$ and the density of the water is $$1000kgm^-3$$.
http://img384.imageshack.us/img384/6944/10062008fj0.png [Broken]
What is the depth x of the oil?

Homework Equations

Pressure = Height x Density x Gravity

The Attempt at a Solution

$$17.5 \times 10^6 = x \times 1000 \times 9.81$$

$$x = \frac{17.5 \times 10^6}{1000 \times 9.81}$$

$$x = 1789.893986m$$

Where did I go wrong?

 

[dirk_mec1]

Both oil and water have a contribution in the pressure:

$$p = \rho g \Delta h$$

So we get:

$$17.5 \cdot 10^6 = 830 \cdot g \cdot x + 1000 \cdot g \cdot(2000-x)$$