Question 64946
You have definitely got the right formula so that's a big plus. The way I worked it, we end up with a quadratic equation which makes it a little more complicated than most word problems. Maybe another tutor will find an easier way. 

As you suggest, we'll use the formula: d=rt or t=d/r


Let r=average speed in banca
Average speed by motor, we are told, is r+4


Now we know that the time spent in the banca (6/r) plus the time spent in the motorboat (15/(r+4)) equals 5.5 hours so our equation to solve is:


(6/r)+15/(r+4)=5.5  multiply both sides by(r)(r+4) to get rid of the fractions and r's in the denominator:


((r)(r+4))(6/r)+(15(r)(r+4))/(r+4)=5.5(r)(r+4) simplifying we get:

6(r+4)+15r=5.5r^2+22r  further simplifying:
6r+24+15r=5.5r^2+22r   which gives:
5.5r^2+22r-21r-24=0
5.5r^2+r-24=0  Now we'll solve this using the quadratic formula:

r=(-1+or-sqrt(1+528))/11
r=(-1+or-(23))/11

r=+22/11 or
r=2mph rowing speed

r also has a negative value which we will discount
r+4=2+4= 6mph speed of the boat

ck
6/2+15/6=5.5
3+2.5=5.5
5.5=5.5

Hope this helps.  Happy holidays-----ptaylor