A boat leaves a dock at 2:00 P.M. and travels due south at a speed of 15 km/h. Another boat has been heading due east at 20 km/h and reaches the same dock at 3:00 P.M. How many minutes past 2:00 P.M. were the boats closest together?

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anonymous · Answer

This question is so confusing. the only thing i could come up with was x is 20 (since it travels for an hour) and x' = 20km/h and y'= 15km/h. 
|dw:1353476410429:dw| --> z^2 = x^2 +y^2  
We implicitly derive that equation so it becomes 2zz'=2xx'+2yy' 
Thats all I have until now, can anyone add to that?