20 minutes is 1/3 hour.
Rick goes 45 mph for 1/3 hour so a distance of
[tex]45 \textrm{ miles/hr} \times \frac 1 3 \textrm{ hr} = 15 \textrm{ miles}[/tex]
At 35 mph that would take
[tex]t =15 \textrm{ miles} / 35 \textrm{ mph}= 3/7 \textrm{ hrs}[/tex]
That's
[tex]3/7 - 1/3 = 9/21 - 7/21 = 2/21 \textrm{ hrs}[/tex]
Answer: 2/21 hours
That's about 5.7 minutes longer
