x = suma intreaga ...
ziua I ... se cheltuieste x/5 (lei) rest = x - x/5 = 4x/5 ;
ziua II ... se cheltuieste 1/3 * 4x/5 adica; 4x/15
rest = 4x/5 - 4x/15=(12x-4x)/15=8x/15 ;
ziua III ... sa cheltuieste 1/2 * 8x/15 = 8x/30 = 4x/15
rest = 4x/15 = 12 (lei)
[tex]\Rightarrow\;x\,=\,\frac{12\cdot15}{4}=45(lei)[/tex]