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Nancy bought 17 cones of ice-cream for her three children. Misha ate twice as many cones as Ana. Dan ate more ice-cream than Ana but less than Misha. How many cones of ice-cream did Dan eat? 
Overseas Competition->Knowledge Point->Calculation Modules->Basic Concepts of Equation->Inequalities->Solving Inequalities
Let x be the number of ice-cream cones that Ana ate.  Then, 2x is the number of cones that Misha ate, and the number of cones that Dan ate is:  17 - x - 2x = 17 - 3x.  It is given that Dan ate more than Misha but less than Ana; therefore,  x \textless{} 17 - 3x \textless{} 2x  4x \textless{} 17 \textless{} 5x  only x=4 satisfies this inequality.  The number of cones that Dan ate is thus 17- 3 x 4 = 5. 