# Thread: simple vectors

1. ## simple vectors

Hi folks,

A ship travels 48 km in a certain time period against a current with an average speed of 6 km/h and 72 km with the stream. The question is what is the speed of the ship when there is no current. I'm stuck!

This should be easy: distance s = v (velocity) x t (time)

48 = (v - 6) t
72 = (v + 6) t

so

48 = vt - 6t
72 = vt + 6t

subtracting

120 = 2vt, so the ship travels 60 km is still water. But I can't figure out how to calculate the velocity!
It seems that going with the stream, the ship makes an extra 12 km, and at 6 km/h this would take 2 hours.
so in still water

60 = vt
60 = v.2

so v = 30 km/h

Sorry, figured the problem out myself. I would delete this but apart from blanking out the entire post, I don't kinow how! If anyone knows how to delete a post, I wopuld be pleased to know.

2. Originally Posted by s_ingram
...

A ship travels 48 km in a certain time period against a current with an average speed of 6 km/h and 72 km with the stream. The question is what is the speed of the ship when there is no current. I'm stuck!

This should be easy: distance s = v (velocity) x t (time)

48 = (v - 6) t
72 = (v + 6) t

...
Even though you got the correct answer I'll do this question nevertheless:

$48 = (v - 6) t~\implies~t=\dfrac{48}{v-6}$
Thus:
$72=(v+6) \cdot \dfrac{48}{v-6}~\implies~72(v-6)=48(v+6)~\implies~24v=720$

Solve for v.

3. Hi Earboth. Yes yours is a neater solution.
By the way, is it possible for me to delete my own post?