A diagram helps.
Diagonal TO divides the rectangle into two triangles of base 10 and height 4. The area of the two of them together (the area of the rectangle) is 10*4 = 40.
The area of rectangle TYOC is 40 square units.
Diagonal TO divides the rectangle into two triangles of base 10 and height 4. The area of the two of them together (the area of the rectangle) is 10*4 = 40.
The area of rectangle TYOC is 40 square units.