Answer:
300 ft²
Step-by-step explanation:
First of all, you have to consider the geometric shapes you could use in order to represent the mainsail. A sail is typically represented in the way of a rhombus, which makes sense considering they are telling you that you have to decompose the shape into rectangles and triangles.
Such triangle can have the following dimensions:
Triangle (Dimensions)
Base = 4ft
Height = 15ft
On the other hand, the rectangle can have the following dimensions:
Rectangle (Dimensions)
15 ft x 20ft
To get the area of the mainsail, you need to add up the sum of the areas of each individual part.
Obtaining each area:
Rectangle: 15x20 = 300ft²
Triangle #1: 1/2(4*5) = 30ft²
Triangle #2: 1/2(4*5) = 30ft²
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