mts1
I agree with you now, I have worked on the problem using both paper and calculator and I agree it should be 50 mph (or rather 51.43 mph).
How ever if you reached my conclusion then hopefully we are both right and something is wrong in what you quoted or the source itself. If we're right and the calculations are right then I don't think you should focus to much on this. As long as you get how you're supposed to do it then I wouldn't worry about it.