### Building Tetrahedra

Can you make a tetrahedron whose faces all have the same perimeter?

A 1 metre cube has one face on the ground and one face against a wall. A 4 metre ladder leans against the wall and just touches the cube. How high is the top of the ladder above the ground?

Four rods are hinged at their ends to form a convex quadrilateral. Investigate the different shapes that the quadrilateral can take. Be patient this problem may be slow to load.

# Last-but-one

##### Age 14 to 16 ShortChallenge Level

Answer: $0$

$99! = 99 \times 98 \times 97 \times ... \times 3 \times 2 \times 1$.

This product contains the numbers $2$, $5$ and $10$, which multiply together to give $100$.

This means $99!$ is divisible by $100$.

The last two digits of $99!$ are therefore $00$, so the last-but-one digit is $0$.

There are plenty of other sets of numbers which together give the required factor of $100$. Some examples include $10$ and $20$, $5$ and $20$ and $2$ and $50$.

This problem is taken from the UKMT Mathematical Challenges.
You can find more short problems, arranged by curriculum topic, in our short problems collection.