valid or invalid contracts

Has a valid contract been formed in the following circumstances? Explain why/ why not (a) Khalid sees an advertisement in the local newspaper which says: " For Sale: Garden chairs, only AED100 for two. Hurry -limited supplies. Send cheque to Garden Furniture Lid. P.O. Box 135'. Khalid sends a cheque but is told the chairs are sold out. (b) Bill writes to George, "You commented that you would like to buy my motorbike. You can have it for AED40,000. If you want it, let me know by the end of the week.' The next day Bill sells his motorbike to John. George hears of the sale and immediately tells Bill he accepts his offer. (c) Abdulla sent out a tender request to all interested businesses in his area for the sale of a fleet of company cars. Talha has sent in the highest bid but Abdulla has refused to sell him the cars. (d) Martin writes to Bob offering to buy his boat for AED5000 and says that if he has not heard from him within a week he will take it that he has accepted. A week has passed and Martin has not heard anything from Bob. (e) On Tuesday Mariam sends a fax to Fatma offering to sell her a gold necklace at a bargain price of AED10.000 for a quick sale. Fatma sends a letter back on the same day accepting the offer. On Wednesday, before Fatma's letter arrives, Mariam agrees to sell the necklace to someone else