x-suma pe care a avut-o mama la inceput
carne: x-x/4=(4x-x)/4=3x/4
fructe: 3x/4-(3x/4)·1/3=3x/4-x/4=2x/4=x/2
lactate: x/2-(x/2)·2/5=x/2-x/5=(5x-2x)/10=3x/10
3x/10=120
x=(120·10)/3=400 lei suma mamei la inceput
b) pt.carne a cheltuit:400/4=100 lei
pt.fructe: (400-100)x1/3=100 lei
pt.lactate: (400-100-100)x2/5=80 lei
verificare
100+100+80+120=400