a department store that bills its charge-account customers once a month has found that if a customer pays promptly one month, the probability is 0.90 that he or she will also pay promptly the next month; however, if a customer does not pay promptly one month, the probability that he or she will pay promptly the next month is only 0.50. what is the probability that a customer who has paid promptly one month will not promptly pay the next three months?

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