In a model of a ship, the mast is 9 cm high, while the mast of the actual ship is 12 m high. If the length of the ship is 28 m, how long is the model ship?

Let the length of model ship be x.

Here length of mast and actual length of ship are in direct proportion.

So, $\frac{12}{9}=\frac{28}{x}.$

$\Rightarrow x=\frac{28\times 9}{12}=21.$

Hence, the length of the model ship is 21 cm.