I've never taken physics, and I'm not even sure this is physics. :S On the hint it mentions, think of freshmen physics, but I honestly have no clue. Could someone help me out on how to even start this problem? Estimate the weight savings if a typical automobile engine (100 kg) is made of aluminum instead of cast iron. Assume the total volume required is the same. (The density of iron is 7.86 g/cm3, the density of aluminum is 2.70 g/cm3). I wanna say that, if the engine is made of iron to start and typically weighs 100kg, then i calculate the volume using density and mass given. Afterwords, you'd find the mass for aluminum using the same volume as cast iron, like the problem states, and it's density. Then just calculate the difference between the two masses. I'm not sure though. :S I'm so confused, so if that didn't make sense, I'm sorry. ~.~ Any help is appreciated, thanks.