s=suma initiala
s = (s/5)+(2s/5)+400 ; numitor comun 5
5s = s+2s+2000 ; 5s-3s = 2000 ; 2s = 2000 ; s = 2000/2 = 1000 lei
x suma initiala
x-x/5-2x/5=400
x-3x/5=400
5x-3x=5*400
2x=2000
=2ooo:2=1000lei
