Basically, this is what you do. Two of your friends go to a ticket booth with a 2 for 1 voucher, and they both get tickets for £35. Then, you go on your own, and buy a full priced ticket for £35. In total, you have spent £70 between you. Split this £70 between 3 of you, and you end up paying £23.33 each. (Sorry about the £33, I had my mind stuck at Chessington's prices).