Nautical Miles Per hour to knots

I have a question,

A ship covers 52 nautical miles in 4h 20min. What is its speed in knots? [1knot = 1 nautical mile per hour]

with the clue given (1 knot = 1 nautical mile per hour) in the question, I could see that 52 nautical miles are directly proportional to 52 knots. But the answer is 12. How to get this answer?

I don't know what to tag this Q. So I tagged it as logic.

• $\frac{52nm}{4h+20min}=\frac{52}{4.333...} \frac{nm}{h}=12\frac{nm}{h}=12knt$
– FWE
Sep 20, 2018 at 12:24
• This question really comes down to the fact that "nautical miles" are not "nautical miles per hour". Can you see how it could get you in trouble if you ignore the two words "per hour"? Sep 20, 2018 at 12:27
• From distance travelled $S$ in a certain time $T$ to speed $V$ (i.e. distance / time) you have to divide the distance travelled by the time : $V = \dfrac S T$. Sep 20, 2018 at 12:46

The answer would be 52 knots if the boat had covered all of that distance in one hour, but it took longer than that, so the speed is slower. (In fact, it took longer by a ratio of $13/3$, so the speed is slower by a ratio of $3/13$.)