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patru paini costa 8 lei .determinati cat costa 9 paini

Răspuns :

Trebuie sa aflam cat costa o paine. 
8:4=2 lei (1 paine)
9x2=18 lei (9 paini)
4paini......................8lei
9paini......................xlei

4/9=8/x
x=9×8/4
x=72/4
x=18 lei (9 paini)