c=cantitatea de cartofi
prima zi s-a vandut 2/9c+40=v1=> cantitatea ramasa dupa prima zi=c-v1=r1
a 2-a zi s-a vandut 4/5r1-20=v2=>
cantitatea ramasa dupa a 2-a zi=r1-v2=r2
a 3-a zi s-a vandut 3/5r2-40=v3
cantitatea ramasa dupa a 3-a zi=r2-v3=r3
r3=840
=>r2-v3=840=> r2-3/5r2+40=840=>2/5r2=800
=>r2=800*5/2=2000
r2=r1-v2=2000=> r1-4/5r1+20=2000=> 1/5r1=2000-20=>r1=1980*5
r1=9900
r1=c-v1=9900=> c-2/9c-40=9900=> 7/9c=9940=> c=9940*9/7=1420*9=
c=12780