There is a special right triangle in this picture ( with angles 30°- 60°- 90° ). Sides of that triangle are: a, a/2 and a√3/2.
a = 10
h = a/2
h = 10/2 = 5
Answer: A ) 5 inches
Steels is equal to x and plastic pipes is equal to y
5x + 50y = $1000
6x + 40y = $100
We already know that the box has an area of 100 sq. inches. All we have to do now is subtract the area of the 4 circles from the area of the box.
The formula for a circle's area is 
. We already know the radius, so one circle has an area of 12.56 units (4π in terms of pi). There are 4 circles, making your final equation 
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