"You have a car that holds 20 litres of petrol, but it has a leak and is losing petrol at a rate of 10ml per minute. If I know I can get 20 miles on 10 litres how far can I travel before I must refuel?" teacher

"Well I'd only go 5 because there's a Halfords just near my house and my mate Dave is a whizz with fixing fuel tanks," student

"But what if you were not at home?" teacher

"I'd look up a station on my satnav," student

"Okay what if you couldn't find one?" teacher

"I'd call the AA," student

"What if your mobile phone didn't work," teacher

" I'd use my other mobile phone I always charge it at night," student

"Well what if there wasn't mobile phone coverage?" asked the teacher

"Well since my mobile phone operator has 98% coverage it means we must be in the Brecon Beacons where I went last year with my dad and the closest station is 7 miles and they even give you a free retest on your MOT there," student

"I give up..." teacher.