A ship leaves on a long voyage. When it is 18 miles from shore, a sea plane, whose speed is ten times that of the ship, is sent to deliver mail. How far from the shore does the sea plane catch up with the ship?

A _______18miles_____B ___x(let)___C
Let the speed of the ship be 1 mile/h and then speed of the plane becomes 10 miles/h.
Let time t is taken to cover the distance x from point B to point C by ship.
Hence, in time t plane will cover (18 + x) miles.

Now,
$$\frac{{\text{x}}}{1} = \frac{{18 + {\text{x}}}}{{10}}$$
Or, 10x = 18 + x
Or, x = 2 miles.
Hence, ship will cover 2 miles in 2 hours and plane will cover 20 miles in same time.

Short method:
Relative speed = (10 - 1) = 9 miles/h.
hence, time taken to catch up the ship is 2 hours so distance covered by the ship = 20 miles.