If I have a 5-pound bowling ball inside an empty 2-pound cardboard box, and I fill the rest of the box with 3 pounds of pure sunlight, how much does the box weigh now?

The bowling ball is 5 pounds, the cardboard box is 2 pounds, and those two together land at 7 pounds on any scale you put them on.

The sunlight is where the question gets interesting. Photons, the particles that make up light, have no rest mass. Zero. A beam of sunlight passing through a container adds no weight you could detect, not even in a laboratory. Physicists will note that photons carry momentum and a form of energy that relates to mass through Einstein's famous equation, but that effect is so vanishingly small that no scale built for bowling balls and cardboard boxes, or any scale you will ever encounter in daily life, would register it. For practical purposes, and for every purpose the question implies, pure sunlight weighs nothing.

So the total is simply 5 plus 2: 7 pounds.

The trick in the question is the phrase "pure sunlight," which sounds like a substance you could pour and measure. It isn't. Light isn't a filling material the way sand or water is. You can flood a room with it and the room doesn't get heavier.