### Pebbles

Place four pebbles on the sand in the form of a square. Keep adding as few pebbles as necessary to double the area. How many extra pebbles are added each time?

Make a set of numbers that use all the digits from 1 to 9, once and once only. Add them up. The result is divisible by 9. Add each of the digits in the new number. What is their sum? Now try some other possibilities for yourself!

### Have You Got It?

Can you explain the strategy for winning this game with any target?

# Factor Trio

##### Stage: 3 Short Challenge Level:
See all short problems arranged by curriculum topic in the short problems collection

Which of the following numbers is the product of exactly $3$ distinct prime numbers?

$45$,   $60$,   $91$,   $105$,   $330$

If you liked this problem, here is an NRICH task which challenges you to use similar mathematical ideas.

This problem is taken from the UKMT Mathematical Challenges.