Jim paddles from one shore of a lake three miles wide at 4 mph, and john paddles from the opposite shore at 5 mph. how long will they travel before they meet?

Distance = Velocity * Time

Now, write the equations for the distance Jim and John each paddle as a function of time.

D_jim = 4 mph * time

D_john = 5 mph * time

And, the total combined distance that they both travel

D_total = D_jim + D_john

D_total = 4 mph * time + 5 mph * time

D_total = (4mph + 5mph) * time = 9 mph * time

Now, since they are both paddling from opposite sides of a river, when they meet, their total combined distance equals the width of the river, or 3 miles

3 miles = 9 mph * time

time = 3 miles / 9 mph = 1/3 hr = 20 minutes <== ANSWER