Question 709372
r, speed of the banca while rowing.
Units are kilometers per hour, and kilometers.  

Vehicle____________rate___________time_______________distance
Motorboat__________r+4____________{{{15/(r+4)}}}___________15
Banca_______________r_____________{{{6/r}}}________________6
TOTAL_____________________________{{{5&1/2}}}____________21


The sum of the times during use of each vehicle is the total traveling time of {{{5&1/2}}} hours.  The only variable in that equation is r, the speed of the banca.  (Rowing done in the banca).  Motorboat speed would be r+4.


{{{15/(r+4)+6/r=5&1/2}}},  find r.