Click on a square to the left of the X-ray bar and move it to the X-ray bar to reveal the arithmetic problem it contains. Once you determine what the answer to the problem is, move it over the square on the right side of the X-ray bar that contains the answer. Once you have the problem over its answer, drop the squire to place it. Move all problems to their answers to complete the level.
Drag the question, move it to the X-ray bar, find the solution and drop it to the right answer.