00:01
So we know he travels a certain distance.
00:03
Let's call it x in three hours.
00:07
And since that's downstream, we have the rate of the boat, let's call it b, plus the rate of the current.
00:14
Let's call that c.
00:17
And going upstream, we have the same distance, x.
00:21
This time we're taking five hours, so our rate is x over five.
00:26
And this time we have the boat speed minus the current, since we're going against the current.
00:31
And we're given a value for the current rate, which is 7.
00:35
So we'll have b plus 7 here, and here we have b minus 7.
00:39
So we have two equations and two unknowns.
00:42
So we're going to solve the system of equations, and we can do so using substitution.
00:46
So looking at our first expression, we can multiply both sides by three...