Answer: 100 ft is the hypotenuse
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Work Shown:
a = 60 and b = 80 are the two known sides
c is the unknown hypotenuse
Applying the pythagorean theorem gets us...
a^2 + b^2 = c^2
60^2 + 80^2 = c^2
3600 + 6400 = c^2
10,000 = c^2
c^2 = 10,000
c = sqrt(10,000)
c = 100
A quick way to see that 100 ft is the missing side is to note that a 6,8,10 right triangle scales up to 60,80,100 after multiplying all three sides by 10.
Or you could start with a 3,4,5 right triangle and multiply all three sides by 20.