a+r=840
r=3a/7
a+3a/7=840
10a/7=840, a=7x840/10=588 trandafiri albi
840-588=252 trandafiri rosii
5x588/6=490 trandafiri albi s-au vandut
5x252/7=180 trandafiri rosii s-au vandut
490+180=670 trandafiri vanduti
840-670=170 trandafiri ramasi
170x5=850 lei costa trandafirii nevanduti