Question

6 vaci si 7 oi au consumat 2 479 kg de fân. Stiind ca o vaca a consumat,in medie, 209 kg de fân, cate kilograme de fân a consumat o oaie ?

Answer 1

1 cate kg de fan au consumat 6 vaci?
6x209=1254
2 cate kg de fanau consumat oile?
2479-1254=1225
3 cate kg de fan a consumat o oaie?
1225:7=175kg

Answer 2

6xv+7xo=2479kg 
v=209kg
6x209+7xo=2479kg
1254kg+7xo=2479kg
7xo=2479kg-1254kg
7xo=1225kg
o=1225kg:7
o=175kg
proba: 6x209kg+7x175kg=1254kg+1225kg=2479kg