The finance director of a hardware wholesaler has asked the accountant to ring each customer five days before their account payment is due, as a means of reducing the number of late payments. As a result of time constraints, however, only 60% of customer's received such a call from the accountant. Of the customers called, 90% pay on time. The company has just received a payment on time from a customer. What is the probability that the accountant called this customer.