The video tutorial teaches how to find unknown lengths and widths of rectangles by applying mathematical reasoning to a known area. It begins with a problem involving a frame and a photo, explaining the concept of area as the space inside a closed shape. The tutorial demonstrates how to calculate the area of a rectangle and use mathematical reasoning to determine unknown dimensions. It includes examples of solving for width when given area and length, and concludes with a recap of the lesson's key points.

Explain how mathematical reasoning can be applied to find unknown dimensions of a rectangle.

What is the unknown width of a rectangle with an area of 42 square inches and a length of 7 inches?