fie x suma depusa => in prima zi: (1/5)x, rest: (4/5)x; in a doua zi: 2/5((4/5)x)=(8/25)x, rest: 3/5((4/5)x)=(12/25)x; in a treia zi: 3/5((12/25)x)=(36/125)x, rest: 2/5((12/25)x)=(24/125)x; in a patra zi: 4/5((24/125)x)=(96/625)x, rest: 1/5((24/125)x)=(24/625)x=120 => 24x=120*625 => x=3125