Some friends went on a vacation to a resort. It poured sown heavily for 13 days. But when it rained in the morning , the afternoon was beautiful and when it rained in the afternoon, the day was followed by clear morning. Altogether there were 11 very nice mornings and 12 very nice afternoons. How many days did the vacation last?
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First, let S stands for Sunny, R stands for Rainy, A stands for Afternoon, M stands for Morning. Originally there can be four types of days: SM+SA, SM+RA, RM+SA, RM+RA. But according to the rule, RM+RM cannot happen, leaving only three types of days. Suppose there are X SM+SA days, Y SM+RA days, Z RM+SA days. So for sunny morning, there are X+Y=11, for sunny afternoon, there are X+Z=12, for rainy day, there are Y+Z=13. Solve the equations, resulting that X=5, Y=6, Z=7.