Answer: 17.5ft
Explanation:
Use Pythagorean theorem
A^2 + B^2 = c^2 where c is the hypotenuse
The ladder leaning against the wall is the hypotenuse.
4^2 + b^2 = 18^2
16 + b^2 = 324
324-16 = b^2
308 = b^2
B = 17.5ft
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