Ship A is 15 miles east of point O and moving west at 20mi/h. Ship B is 60 miles south of O and moving north at 15mi/h. a)Are they approaching or seperating after 1 hour and at what rate? b)after 3hrs?

Let D=distance between the ships at time t

a)mi/h

b)mi/h

The book agrees with me on part A, but on part B it says the answer should be mi/h. Am I missing something obvious or is the book wrong here?