I think the keyword is 'rate' of tax paid.
So essentially, the answer is (250/5000)/(480/12000)
which is (250/5000) * (12000/480) = 5/4.
Just a question.... should it not be this way ???
The tax in excess of 12000 will be 480 + 250 = 730
so the rate of tax in excess should be (730/17000) / (480/12000)
which makes it 73/(4x17) , however it does not match the answer.