Mr. Black gave his wife money every Friday evening, but she always spent it before the next Wednesday. For the next three days she had none. Every Tuesday evening Mr. Black asked her, "But what did you spend all that money on?" and she always answered, "I don't know." One Friday Mr. Black brought home an exercise book and a pencil and gave them to his wife along with the money. "Now look!" he said to her. "When you get money from me, write it down on this page, and on the next page write down what happens to the money." When Mr. Black came home the next Tuesday, his wife came to him and showed him the book. "I have done what you told me," she said happily. On the first page she had written "Friday, 28th June. I got PS18 from John," and on the next page, "Tuesday, 2nd July. I have spent it all." By   _   evening there was no more money left for Mrs. Black.
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