Suma totala:
|-------------------------------|
in prima zi: jumatate
|--------------|
in a doua zi o treime din rest:
|----|
a treia zi 1/7 din rest
|--|
ramane:
|-----| = 420
suma totala X este:
X=420+(1/7)x+(1/3)X+(1/2)X
aducem la acelasi numitor 42 =>
42x=42*420+6x+14x+21x=>
42x-6x--14x-21x=17640
x=17640