Answer:
see attached
Explanation:
We cannot really do the construction for you, but we can tell you how it needs to go. The three sides of the triangle will have a length in cm that is equal to the given length in km divided by 20.
AB = 6 cm
BC = 8 cm
AC = 9 cm
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The area each station can be heard will be a circle of radius (100 km)/(20 km) cm = 5 cm. The overlap of those circles is the area you are to shade.
The attached diagram should give you the general idea. (Scale units are cm.)
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Additional comment
It works well to find B as the intersection of arcs of radius 6 cm and 8 cm, drawn from centers A and C, respectively.