Two lumps of putty are moving in opposite directions, each one having a speed of $30.0 \mathrm{~m} / \mathrm{s}$. They collide and stick together. After the collision the combined lumps are at rest. If the mass of each lump was $1.00 \mathrm{~kg}$ before the collision, and no energy is lost to the environment, what is the change in mass of the system due to the collision?