The separation between the ship and the seaplane is 18 miles. And since the two are travelling in the same direction, the relative speed would be 9 times the speed of the ship (If speed of ship is x miles/hour, speed of seaplane would be 10x and 10x − x = 9x). Hence to catch up with the ship the seaplane would take
18
9x
=
2
x
hours. Now, the ship covers x miles in an hour, so in
2
x
hours it would cover 2 miles. So, when the seaplane catches up with the ship, it would be 18 + 2 = 20 miles from the shore.