Error calculating chargeable minutes based on hourly rate

Error calculating chargeable minutes based on hourly rate when producing invoices. I've had the following calculated based on an hourly rate of GBP 35: 0:10 hours - 5.95 (should be 5.83), 0:20 hours - 11.55 (should be 11.67), 1:05 - 37.80 (should be 37.92), 0:15 - 8.75 (correct). I can understand that the system might be trying to round to, say, the nearest 5p, but these totals are significantly out. Any ideas?