$\frac{{{V}_{S.AMN}}}{{{V}_{S.ABC}}}=\frac{SM}{SB}.\frac{SN}{SC}.\frac{SA}{SA}=\frac{1}{4}\Rightarrow {{V}_{S.AMN}}=\frac{1}{4}{{V}_{S.ABC}}=\frac{1}{8}{{V}_{S.ABCD}}$

$\frac{{{V}_{S.AND}}}{{{V}_{S.ACD}}}=\frac{SN}{SC}=\frac{1}{2}\Rightarrow {{V}_{S.AND}}=\frac{1}{2}{{V}_{S.ACD}}=\frac{1}{4}{{V}_{S.ABCD}}$

$\Rightarrow VS.AMND={{V}_{S.AMN}}+{{V}_{S.AND}}=\frac{3}{8}{{V}_{S.ABCD}}$