To find the total distance that the four friends swim in a relay, you can multiply the distance each friend swims by the number of friends.
Each friend swims \( \frac{3}{5} \) of a mile. Therefore, the total distance for 4 friends is:
\[ 4 \times \frac{3}{5} = \frac{12}{5} \text{ miles} \]
Now, let's convert \( \frac{12}{5} \) to a mixed number:
\[ \frac{12}{5} = 2 \frac{2}{5} \text{ miles} \]
So, the total distance they swim is \( 2 \frac{2}{5} \) miles.
The correct response is: 2 2/5 miles.