I have a math test on Thursday, but I don't understand how to do these two questions. Can someone help please, and explain why you would do what you do?

Thanks

1. The back yard of a home is a rectangle 15 m by 20 m. A garden of uniform width is to be built around the edge leaving a grass area inside. The area of the grass is to be the same as the area of the garden. What are the dimensions of the garden?

2. If the average speed of a light place had been 40km/h less, the plane would have take one hour longer to fly 1200 km. What was the speed of the plane?

Much appreicated : )