The volume of a cylinder is calculated by:

With your problem you have r = 10, h = 10. That means the maximum volume is:

The actual volume is three quarter of the maxium volume:

that means the surface of the oil is 7.5 m above ground. So the height above the leak is 3 m:

...

2. Calculate the volume which has already leaked from the tank:

Since 5000 L = 5 m³ you have to solve for t:

Use logarithms to calculate t. I've got t ≈ 0.0505 h and

**that are nearly 3 min** (This result seems a little funny to me

I assume that I've made a silly mistake with my calculations. So check!