Find a) the total distance convered in km and b)the average speed for the complete trip in m/s.

An automobile travels at the rate of 25 km/h for 4.0 minutes, then at 50.0 km/h for 8.0 minutes and finally 20.0 km/h for 2.0 minutes. Find a) the total distance convered in km and b)the average speed for the complete trip in m/s.

I got the first answer correct: 9.00 km
I did this by converting each km/h into km/minute which converted to .417 km/min, .833 km/min, and .333 km/min then multiplying each one by its respective amount of minutes.
I keep trying to calculate part b of this problem but it says the correct answer is 10.7 m/s (or 11m/s) and none of the answers I've come up with have given me this. Please help, I know I'm probably making a dumb mistake, I just want to know how to correctly work this problem out.

Roxy-
Don't always assume YOU are wrong. Answer books ( and proof-readers) are not infallible.
Let's work:

9 km in how long? 4 min. + 8 min. + 2 min. = 14 min. This is 9000 m in 14 minutes, 9000 m in 840 seconds. I get 10.7 m/s.

Well, I'd love to be supportive - and I hate answer books - but, this time the book is correct.

Richard

Average speed = total distance / total time.

Take each leg and calculate total distance and time, then add up the numbers and do the above calculation

Usual mistake is to average the speeds, but that is not the correct way.

Work in km/h, will be much easier.
25 * 4/60 + 50 * 8/60 + 20 * 2/60 = 540/60 = 9km

average speed = 9/((4+8+2)/60) = 38.57 [km/h]
38.57[km/h] = 38570[m]/3600[s] = 10.71 [m/s]

Obviously you can calculate the average as 9[km]/14[min] = 9000[m]/840[s] = 10.71[m/s]