Way to find volume of the solid A solid has a square base of side $s$ . The upper edge is parallel to the base and has length $2s$. All other edges have length $s$ . What is the volume of the solid ? 
NB : The volume of the tetrahedron with all sides length l is $ V = \dfrac{\sqrt2}{12}l^3$

We can divide the solid in two tetrahedra and a square pyramid. See fig.1.

Fig.1
The pyramid height $GH$ has measure $x$, and the measure of $DB$ is the double of $x$. 
(Check it out).
Using pythagoras in $\triangle ABD$ we get:
$$x=\frac{s \sqrt{2}}{2}$$
We know that the volume of the solid $V$ is:
$$V=2V_{tetra}+V_{pyr}$$
where $V_{tetra}$ is the volume of each tetrahedron and $V_{pyr}$ is the volume of the square pyramid.
Calculating each term we have:
$$V=2 \frac{\sqrt{2}}{12}s^3+\frac{1}{3}s^2 \frac{s \sqrt{2}}{2} \Rightarrow$$
$$V= \frac{\sqrt{2}}{3}s^3$$
