This question is a little dodgy because we don't know how much a glass bottle weighs. I'll assume that one bottle is one kilogram.
This means that 1kg of glass can power the oven for 7.5 mins. Adam needs to power the oven for 6 hours, or 360 minutes, therefore he needs (360/7.5)kg of glass, which is 48 kg.
Glass costs 5p per kg, so 48kg will cost 240p
So the glass needed costs 240p or £2.40 (I assume the p meant pence.)
Hope this helped, it might be wrong given the glass may not be 1kg per bottle.
