Net force and acceleration

[Sko]

I'm doing some problems I found here:

http://library.thinkquest.org/10796/ch4/ch4q.htm

A car is moving at a constant velocity of 20 km/h (5.56 m/s). How much net force is required to raise its velocity to 50 km/h (13.89 m/s) in 30 seconds? Suppose the car has a mass of 150 kg.

I used F= (150kg)([13.89m/s-5.56m/s]/30s)
and got 41.65 N but apparently that's wrong. What am I doing wrong?

[Doc Al]

The only thing you're doing wrong is writing your answer with too many significant digits. [:)] Round off to two digits.

[Sko]

Ha! My phsycis teacher doesn't make us use significant figures (chem teacher did last year) so I completely forgot to do that :P