It appears that both of these questions have some errors that could lead to confusion.  Sorry about that.  Here are some explanations.

9.24: The question should mention that Derrick and Jimmy meet at some place along the route.  When they meet, their combined travel time is $41$ hours.  (If we don't mention that they meet, and both travel the full distance between NY and LA, then the fact that Derrick starting off 2 hours later than Jimmy will not make sense, since it doesn't affect the total time.)

9.27: The posted answer key is incorrect.  The actual answer should be "approximately $6.63$ m/s".  If you got this answer, then good job!  If not, please show how you did it, and we will see whether it can be corrected.