tax is paid as a percentace of earnings. So, as an example, if you were taxed 10% of 17500, yur tax would be10 pounds in every 100 pounds = �1750
if your wages went up to 24000 then your tax would be (again) 10 pouns in every 100 pounds = �2400
so as you see when you pay a percentage, if you get more you pay more. Of course those figures given above bear no relation to real life, but are just an example to show what a percentage is and how the absolute figure will go up if the wages go up