x=suprafata totala
prima zi x/3-20 ramane x-(x/3-20)=x-x/3+20=2x/3+20
a doua zi 2(2x/3+20)/7+10=2(2x/21+20/7)+10=4x/21+40/7+10 insamantat
ramane dupa a doua zi 2x/3+20-(4x/21+40/7+10)=2x/3+20-4x/21-40/7-10=
2x/3-4x/21-40/7+10=10x/21+30/7
a treia zi (10x/21+30/7)/2+5=5x/21+15/7+5, deci
x=x/3-20+4x/21+40/7+10+5x/21+15/7+5+45
x=x/3+9x/21+55/7+40 numitor comun 21
21x=7x+9x+165+840
5x=1005, x=1005/5=201 ha
