A merchant woman buys 7 apples for 1 denaro, and she sells 6 for 1 denaro, and she buys 8 pears for 1 denaro, and sells 9 for 1 denaro, and she invests 10 denaro and makes a profit of 1 denaro. How much did she invest in apples; how much in pears?
Liber Abaci, Leonardo of Pisa,1202
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