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 .