Question 931612
r+ c = 20fps    40ft/2sec
<u>r- c = 5ftps</u>  40ft/(8sec)
2r = 25
r = 12.5
c = 7.5fps 
Re TY 
 (in the above had both ft and sec to deal with converting to mph)
as (as 1*3600 = 1 hr)
c = 7.5(3600)ft/hr) AND {{{((7.5*3600)/5280)fph}}} = mph
.......
8 miles on a river in 20 minutes 
(as 20*3 = 60min)
Yes. 8*3= 24mph  
Good Work.