Area Mazes - fun puzzles by a master puzzle inventor

Menseki meiro means "area mazes." It's a new puzzle genre that challenges you to derive the length or area of rectangles based on the given dimensions of neighboring rectangles. I got an advance copy of Area Mazes, with 100 puzzles. It's addictive. They start out easy. The later ones are very difficult.

Here are the rules:

Here's an easy one:

Here's a hard one: