Using appropriate legal terms, explain whether a valid contract has been made in the following situation:

Tom runs a business and Sue is an auditor. Tom asks Sue, "What's your rate for book keeping services?" Sue says, "$200 an hour but as a special deal we'll charge you $150 for the first 10 hours work." Tom says, "Do you work weekends?" Sue replies, "Yes but not public holidays." Tom says, "OK, I accept but I will expect you to work over the Labour Day weekend."