Clear["Global`*"]

"Fixed values";

Va=2; Ka=3; Vb=7; Kb=10; Vcfw=6; Kcfw=0.5; Vcr=4; Kcr=3; Vd=2; Kd=5; Ve=1; Ke=4; Vf=3; Kf=3;

"Unknowns values";

" a;b;c;d;e;f ";

NSolve[f== Kf*Ve*e/(Vf*Ke+(Vf-Ve)*e)&&

b==Kb*Va*a/(Vb*Ka+(Vb-Va)*a)&&

c==-d*Kfw*(d*Vd+Kr*Vd+d*Vr+Kd*Vr)/(Kr*(d*Vd-d*Vfw-Kd*Vfw))&&

d== (-b*c*Kr*Vb-b*Kfw*Kr*Vb+b*c*Kr*Vfw+c*Kb*Kr*Vfw)/(Kfw*(b*Vb+b*Vr+Kb*Vr))&&

e==Ke*Vd*d/(Ve*Kd+(Ve-Vd)*d)&&

a+b+c+d+e+f==1 &&

a>0 && b>0 && c>0 && d>0 && e>0 && f>0

,{a,b,c,d,e,f}]

{{a->0.137249,b->0.126577,c->0.0337561,d->0.228748,e->0.383544,f->0.090125}}