Answer :
The space between them is shrinking at the rate of (4 + 5) = 9 mph.
The lake is 3 miles wide, so the space between them starts out at
3 miles, and shrinks to zero in 3/9 = 1/3 of an hour = 20 minutes.
The lake is 3 miles wide, so the space between them starts out at
3 miles, and shrinks to zero in 3/9 = 1/3 of an hour = 20 minutes.
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
