### 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?

### Days and Dates

Investigate how you can work out what day of the week your birthday will be on next year, and the year after...

# Repeat Product

##### Stage: 4 Short Challenge Level:

Notice that
$$RRR=111 \times R$$
and
$$PQPQ=1000 \times P+100 \times Q +10\times P +Q=101 \times (10\times P+Q)$$
so $PQPQ\times RRR=111 \times 101 \times R\times(10 \times P+Q)$.

And we have $639027=PQPQ\times RRR=111 \times 101 \times R\times(10 \times P+Q)$ so we can divide both sides by $111\times101$ to give $$57=R\times(10\times P+Q)\;.$$

The only factors of $57$ are $1,3,19$ and $57$.

$R$ must divide $57$ and because $R$ must be a single digit number it can either be $1$ or $3$.

If $R$ is $3$ then $10\times P+Q=19$ so $P=1$ and $Q=9$.

If $R$ is $1$ then $10\times P+Q=57$ so $P=5$ and $Q=7$.

So there are two solutions (check that these both work) and in both cases $P+Q+R=13$.

This problem is taken from the UKMT Mathematical Challenges.