I'm working from a study guide and i got a question asking:
a car is driven 3km east for 5 mins, 4km for 8 mins and 3km for 2 mins.
calculate the average speed in km h-1
so far i have done the calculations (the next part of the question is average velocity, but i'm sorta confident to work that one out)
3km+4km+3km = 10km
5+8+2 = 15 minutes
my question is: for the equation for speed i use distance traveled/time taken
do i have to use seconds for the time or minutes in this case. and how do i convert meter per second to km per hour.

merci : d

1).  You're always free to use whatever units you like at the moment. Just be sure to include the units in your answer.  

BUT ... If the question TELLS you what the units of your answer should be, then that's the units your answer must have.

That's what you've got here.  The question said "calculate the average speed in Km h-1."  That's just a fancy way to write ' km/hour '.  So there's the units of your answer.  The data given is in minutes, so you'll need to do some work to get the answer in km/hour .

The equation for speed is  

Speed = (distance covered) / (time to cover the distance"

When you plug a number into that equation, keep the units hooked to it, and keep the units all the way through your work.

Speed = (10 km) / (15 minutes)

Speed = (10/15) (km/minutes)

Speed = 2/3 km/min

There's the speed.  But the units are not what the question wants to see.  You could convert the units into miles per hour, feet per second, smoots per day, furlongs per fortnight, or nano-inches per eon.  But you're smart enough to know that you won't get any credit for the answer if it doesn't have the units that the question commanded you to use.  So you have to convert the 'km/min' into 'km/hour'.

Just multiply your answer by (60min / hour) .  That fraction is equal to ' 1 ', because the numerator and denominator are equal, so it won't CHANGE your answer by mutiplying.  But you WILL convert the units of the answer.

Speed = (2/3 km/min) x (60 min/hour)

Speed = (2/3 x 60) x (km/min x min/hr)

Speed = (2/3 x 60) x (km/hr)

Speed = 40 km/hr .