John is a 15-year-old boy living in London. Briefly explain to John the legal position arising from the following scenarios:

(a) In the morning John offers to wash his dad's car for 10. Having washed the car, his dad tells him that he changed his mind and does not pay John!

(b) Later in the afternoon, John is out at the high street, when he sees in a shop his favourite skateboard being offered for 50, i.e. half the normal price. He enters the shop and asks to buy it at that price. The shopkeeper tells him that there hasbeen a mistake on the price labelling and the price is 100 for the skateboard.