### 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?

# Counting Factors

##### Stage: 3 Challenge Level:

Charlie wants to know how many factors 360 has.
How would you work it out?

Click below to see what Alison did.

Alison divided 360 by each number in turn to see if it's a factor, and wrote down the factor pairs:

(1, 360)
(2, 180
(3, 120)
(4, 90)
(5, 72)
(6, 60)
(8, 45)
(9, 40)
(10, 36)
(12, 30)
(15, 24)
(18, 20)

"I can stop there, because the next factor would be 20 and I've already got that. So there are 24 factors."

Charlie thought about it in a different way. Click below to see what he did.

Charlie started by working out the prime factorisation of 360.

\begin{align} 360 &= 2 \times 180 \\ &= 2 \times 2 \times 90 \\ &= 2 \times 2 \times 2 \times 45 \\ &= 2 \times 2 \times 2 \times 3 \times 15 \\ &= 2 \times 2 \times 2 \times 3 \times 3 \times 5 \end{align}

So $360 = 2^3 \times 3^2 \times 5$.

Then he used a table to find all the possible combinations of the prime factors.

$2^0$
$3^0$
 $5^0$ $5^1$
$3^1$
 $5^0$ $5^1$
$3^2$
 $5^0$ $5^1$
$2^1$
$3^0$
 $5^0$ $5^1$
$3^1$
 $5^0$ $5^1$
$3^2$
 $5^0$ $5^1$
$2^2$
$3^0$
 $5^0$ $5^1$
$3^1$
 $5^0$ $5^1$
$3^2$
 $5^0$ $5^1$
$2^3$
$3^0$
 $5^0$ $5^1$
$3^1$
 $5^0$ $5^1$
$3^2$
 $5^0$ $5^1$

So the top branch gives us $2^0 \times 3^0 \times 5^0 =1$
and the eleventh branch gives us $2^1 \times 3^2 \times 5^0 = 18$

When she saw Charlie's method, Alison said "There must be lots of numbers which have exactly 24 factors!"

Charlie and Alison think all of these numbers have exactly 24 factors. Can you see why?

$25725 = 5^2 \times 3^1 \times 7^3$
$217503 = 11^1 \times 13^3 \times 3^2$
$312500 = 5^7 \times 2^2$
$690625 = 17^1 \times 13^1 \times 5^5$
$94143178827 = 3^{23}$

Here are some questions to consider:

How can I find a number with exactly 14 factors?
How can I find the smallest such number?

How can I find a number with exactly 15 factors?
How can I find the smallest such number?

How can I find a number with exactly 18 factors?
How can I find the smallest such number?

Which numbers have an odd number of factors?

Extension:

What is the smallest number with exactly 100 factors?

Which number less than 1000 has the most factors?