are my answers below correct to part a and part b?

15-6

Currently, it takes five days from the time customers mail payments until Shark Bait Rafts (SBR) deposits them. SBR would like to set up a lockbox collection system that would reduce this collection float by three days. If SBR receives an average of $2,500 in payments per day and its opportunity cost is 18 percent, how much should SBR be willing to pay each month for the lockbox system?

a5 days receive payments and deposit; Lockbox would reduce this time by 3 days; $2,500 per day in payments; Opportunity cost is 18 percent

$2,500 x 5 = $12,500 x .18 = $2,250

$2,500 x 2 =$5,000 x .18 = $900

b) SBR should be willing to pay $1,350 each month for the lockbox system?