he will pay 112.50 for just the 15 more sun visors (not including the 12 he already bought), for 16 he'd pay $120.
to figure this out you find out how much just one sun visor is so you divide the $90 by 12 sun visors and get 7.50, then you times 7.50 by 15 to get the cost of buying 15 sun visors, same scenario with 16 sun visors... hope this helps.