Apaleontologist estimates that when a particular rock formed, it contained 12 mg of the radioactive isotope potassium-40, which has a half-life of 1.26 billion years. the rock now contains 3 mg of the isotope. about how old is the rock? enter in billions of years.

Asolid cylinder and a cylindrical shell have the same mass, same radius, and turn on frictionless, horizontal axles. the cylindrical shell has light-weight spokes connecting the shell to the axle. a rope is wrapped around each cylinder and tied to blocks of equal masses that are held the same height above the ground. both blocks are released simultaneously. the ropes do not slip. which block hits the ground first? or is it a tie?