A wizard and a sorceress are dueling in a magical forest. Each day, the wizard casts 3 spells, and the sorceress counters with 4 spells of her own. They are limited by the magical energy in the forest, which can sustain a total of 140 spells. How many days can they duel before exhausting the magical energy of the forest? Let's think step by step and output the final answer within \boxed{}.
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