54. In the figure shown below, B is between A and D, the length of AB is 30 miles, the length of BC is 40 miles, and the measure of ∠CBD is 60°. To the nearest mile, what is the length of AC ?

(Note: sin60°=sin120°≈0.866; cos60°=0.5; cos120°=-0.5)

(Note: For a triangle with sides of length a， b, and c that are opposite ∠A，∠B，and ∠C, respectively，c² = a² + b²- 2ab cos C, and .)