A space diagonal of a cube joins opposite vertices and passes through its centre, as illustrated. Cube $A$ has a space diagonal twice as long as that of cube $B$. How many times the volume of $B$ is the volume of $A$?
Triangle $ABC$ is isosceles at $A$. The internal bisector $BD$ of $\angle ABC$ meets $AC$ at $D$. Its altitude $AH$ satisfies $AH=\tfrac12BD$. Find $\angle BAC$.
Jerry must run two laps of a badminton court. His average speed on the first lap is $200$ m/min. What average speed is needed on the second lap to achieve an overall average speed of $240$ m/min?
In the figure, $\triangle ABC$ is right-angled at $A$ and $\angle ACB=30^\circ$. Triangles $ABE,ACF,BCG$ are equilateral. Given $S_{ABE}+S_{ABC}=12\text{ cm}^2$, find $S_{ACF}+S_{BCG}$.
In square $ABCD$, $M$ is the midpoint of $AD$, and $MN$ is perpendicular to $AC$ with $N\in AC$. Find the ratio of the shaded area $S_{MNC}$ to the area of the square.