The most likely answer to this problem would be (1) more mass and more inertia.
A 15-kilogram cart at rest and a 5-kilogram box would make up a 20-kilogram cart and box that is at rest on a horizontal surface. The mass changed into something more, of course, as a result of combining the two object into one and by combining the two objects' mass, the inertia that it previously possessed as a cart by itself was increased when the inertia of the box was also combined to the cart.
