A fish swims across the width of an 18 m wide river in 6 s. What is the speed of the fish?

We know that, 

                    $Speed\ =\ \frac{Distance}{Time}$

given, distance = 18m and time = 6s

Therefore, $Speed\ =\ \frac{Distance}{Time} \ =\ \frac{18}{6} \ =\ 3\ m/s$