15 In the rectangular prism $A B C D-A_{1} B_{1} C_{1} D_{1}$, $A B=2, A A_{1}=A D=1$, points $E, F, G$ are the midpoints of edges $A A_{1}, C_{1} D_{1}, B C$ respectively, then the volume of the tetrahedron $B_{1}-$ $E F G$ is $\qquad$
(15) $\frac{3}{8}$