Actual height of chimney=12m=12*100 cm=1200cm

height of model of chimney=9cm

Using unitary method

If actual height of chimney is 1cm then height of model of chimney=9/1200 cm =0.0075cm

So height of model of a object of 1cm height=0.0075cm

Now we have been given that height of factory is 28m=2800cm

the height of model of factory= height of factory x height of model of height 1cm

=2800*0.0075cm=21cm=0.21m

height of model of factory=0.21m