vectors in mechanics question

Please can someone tell me if my answers are correct for the following questions because they do not agree with the ones in the back of my text book.

(in this question unit vectors i andj are due east and due north respectively.
At noon a ship S is 600m due north of an observation point O and a speedboat B is 120m due north of the same point. The ship S has a constant velocity (7i + 8j) m/s and the speedboat B has a constant velocity of (7i + 24 j) m/s.

a) show that S and B will collide and find the time when this collision occurs and the position vector of the point of collision

if the ships collide they will have the same position vector at the same time. If rs = rb then:

600j + (7i + 8j)t = 120j + (7i + 24j)t

or:

(7t)i + (600 + 8t)j = (7t)i + (120 + 24t)j

equating the j parts gives:

600 + 8t = 120 + 24t
t = 15

the text book says t = 30 and makes my position vector incorrect, am I wrong if so where did i go wrong?

b) in order to prevent a collision, 15s after noon S changes its velocity to (7i + 30j) m/s
find the distance between S and B 30s after noon

I keep getting 330m but the back of the book says 90m???

Please help