The following transactions are July activities of Bill's Extreme Bowling, Incorporated, which operates several bowling centers. If revenue is to be recognized in July, indicate the amount.

$\backslash$table$[[$Activity$,$Amount$],[$a$.$ Bill's collected $$20,700$ from customers for services related to games played in July.,$],[\backslash$table$[[$b$.$ Bill's billed a customer for $$375$ for a party held at the center on the last day of July. The bill is$],[$to be paid in August.$]],],[\backslash$table$[[$c$.$ The men's and women's bowling leagues gave Bill's advance payments totaling $$2,250$ for the$],[$fall season that starts in September.$]],],[$d$.$ Bill's received $$2,800$ from credit sales made to customers last month $($in June$).,]]$