High Fare

Sardar asked a bus conductor how much the bus fare into the city was.
"Fifteen pence,' said the conductor.
Sardar thought this was a bit much so he decided to run behind the bus for few stops.
'How much is the fare now?' he panted, after running three stops behind the bus.
'Still fifteen pence,' said the conductor.
Sardar ran three further stops behind the bus and could just about manage to ask the conductor again what the fare was now.
'Twenty pence,'said the conductor. 'You're running in wrong direction.'