Read this and answer the question. If the question is unanswerable, say "unanswerable".

In 2003, a congestion charge was introduced to reduce traffic volumes in the city centre. With a few exceptions, motorists are required to pay 10 per day to drive within a defined zone encompassing much of central London. Motorists who are residents of the defined zone can buy a greatly reduced season pass. London government initially expected the Congestion Charge Zone to increase daily peak period Underground and bus users by 20,000 people, reduce road traffic by 10 to 15 per cent, increase traffic speeds by 10 to 15 per cent, and reduce queues by 20 to 30 per cent. Over the course of several years, the average number of cars entering the centre of London on a weekday was reduced from 195,000 to 125,000 cars a 35-per-cent reduction of vehicles driven per day.

What is the daily cost for most drivers to operate their cars within a given zone in the center of London?
10