So I don't know the height of Mt. Kilimanjaro off the top of my head, but I did know the height of Everest is 8848.13m above sea level (the things you learn in Chinese School!). Assume a roughly conical shape; I believe I took the mass of a meter cubed of dirt to be something like 100kg? A friend of mine did this out once using numbers he looked up and assuming igneous rock (since it's volcanic) and got something like 4e11 kg, but even my interviewer admitted he had no idea what the real answer would be.