Answer:
Step-by-step explanation:
There are a couple of things to know before we set up the table for this problem. First, we need to remember that if the boat is traveling against the current, that the current is going to slow the boat down; likewise, going with the current will speed the boat up. Second, we need to realize that if the boat's distance against the current is the same as the distance with the current, then the distances in the equations will be set equal to one another.
Set up the table with U (against the current) and D (with the current) on the outside and fill in what we are given:
           d      =      r       x       t
U Â Â
D
This is the table. Now we will fill it in:
           d      =      r        x       t
U                     r - 3       x       3
D                     r + 3       x      1.8
The formula for the distance equation is at the top of the table as d = rt. We already decided that the distances that the boat traveled upstream and downstream were the same so we will set those d values equal to each other, but we first need to get the d value for each row. If distance = rate times time, then for the first row:
d = (r - 3)3 and
d = 3r - 9. For the second row:
d = (r + 3)1.8 and
d = 1.8r + 5.4. Setting them equal to each other:
3r - 9 = 1.8r + 5.4 and
1.2r = 14.4 so
r = 12 mph
