not sure I'll get this 100% right but I'm pretty confident.
by using r=d/t you're saying what you need (r) is got by putting distance (d) over time (t) so this equation should be as simple as doing 286/5.5 ( .5 comes from the " half hour" ) which gives you 52 miles per hour it's already in miles per hour so you don't have to change anything else from there