The cooperative Gaudrette buys 5,000 bags of fertiliser at 30 each, that is 150,000 excluding tax. The contract provides for a rebate of 6,000 on this batch, earned upon delivery, and transport to its silos costs 4,500. Its controller computes a unit cost of 30.90, that is (150,000 + 4,500) / 5,000. What is the error?