Răspuns:
400 lei a cheltuit Oana in cele trei zile
Explicație pas cu pas:
Notam cu x suma cheltuita de Oana in cele trei zile
[tex]prima \: zi \: cheltuieste\: x:2\\\\\:\:\:rest \: dupa \: ce \: a\: cheltuit\: in\:prima\:zi\: x - x:2 = \boxed{ x:2}\\\\cat\:a\:cheltuit\: a\:doua\:zi\: (x:2):2 =\boxed{ x:4}\\\\cat\:a\:cheltuit\: a\:treia\:zi\: 100\\\\x:2 +x:4 +100 = x\:\:\:\:\inmultim\:toata\:ecuatia\:cu\: 4\\\\ 2x +x +400 = 4x\\\\3x+400=4x\\\\400 = 4x-3x\\\\\boxed{x=400}[/tex]
