Answer: 10
Explanation:
As shown below, ∠ABD = ∠DAC = x.
That means tan(x) = 5 by AD and that tan(x) = AD by 20
You can set those two equations equal to each other to find the length of AD:
5 by AD = AD by 20
5 = AD square by 20
100 = AD square
AD = Square root 100 = 10