BE$17\mathrm{.}1($LO $1)$ Leno Computers manufactures tablet computers for sale to retailers such as Fallon Electronics. Recently, Leno sold and delivered $200$ tablet computers to Fallon for $$20,000$ on January $5,2025.$ Fallon has agreed to pay for the $200$ tablet computers within $30$ days. Fallon has a good credit rating and should have no difficulty in making payment to Leno. $($a$)$ Explain whether a valid contract exists between Leno Computers and Fallon Electronics. $($b$)$ Assuming that Leno Computers has not yet delivered the tablet computers to Fallon Electronics, what might cause a valid contract not to exist between Leno and Fallon?