He pays $16.52 ((89.95+257)- (89.95+257)/1.05)) sales tax for a chair and a couch. This sales tax amount is calculated assuming that Gerald paid the after-tax price for the items. The sales tax amount can be calculated after we acquired the before-tax amount of the items and then, the sales tax amount can be acquired by subtracting the after-tax price with the before-tax price.