Distance, Rate, Time question?

First, convert everything to the same set of units. Since you're using mph for the speed, use hours and miles for time and distance. So, now the boat takes 0.1 hours longer on the way back. Let's define some variables: t := time on the way to the island, d := distance to the island. As you can see, the time for the way back should be t+0.1.

So, now, given that d=rt you should be able to get your equations.

d = 15(t) on the way out

d = 12(t+0.1) on the way back

combing these two by substitution:

d = 15(t)

d/15 = t

- then subst -

d = 12(t+0.1)

d = 12(d/15 + .1)

d/12 = d/15 + .1

d = 12d/15 + 1.2

d - 12d/15 = 1.2

3d/15 = 1.2

d/5 = 1.2

d = 6

Six miles!