KöMaL 2015 K478

Farmer Thomas wanted to buy 4 metres of chain that costs 210 forints (HUF, Hungarian currency) a metre. The shop assistant tried to talk him into buying all the 10 metres they have in stock, but Farmer Thomas insisted on buying 4 metres only. However, he noticed that the assistant, on purpose, made a mistake in measuring the 4 metres, and cut off a shorter piece. Therefore he decided to ask for the other piece instead, which the assistant had to sell him for the price of 6 metres in order to avoid being caught cheating. Had he not noticed the cheating, it would have cost Farmer Thomas 14/9 as much to buy a metre of chain as it actually cost with this clever manoeuvre. How many metres of chain did Thomas get?