Imagine a 10 gram rock thrown at the same time as a 20 kilogram rock both travel in a straight line. how does intertia a compare for the 2 rocks?

As clearly stated in this problem the two object has different amount of mass. M1 = 10 grams, and M2 = 20 Kg. Larger mass would have a bigger Inertia. Inertia is the resistance of any physical object to any change, may it be speed or direction. It would require greater amount of  force to throw M2 than M1. The force or pull of gravity in each object is the same, but the massive object would have a greater resistance to change in it's position, speed, and direction.