The first three stages of a pattern are shown below - Junior Cycle Mathematics - Question 11 - 2014

Based on the pattern observed:

Number of Shaded Discs	Number of White Discs
1	5
2	7
3	9
4	11
5	13
6	15

The pattern shows that as the number of shaded discs increases by 1, the number of white discs increases by 2.

Using the established pattern, we see that the number of white discs can be represented by the formula:

$ext{White discs} = 3 + 2n$

where $n$ is the number of shaded discs. To find $n$ when there are 21 white discs:

$21 = 3 + 2n$

Subtracting 3 from both sides, we have:

$18 = 2n$

Dividing both sides by 2, we get:

$n = 9$

Thus, there are 9 shaded discs.

The relationship can be described by the equation:

$ext{Shaded discs} = n$ $ext{White discs} = 3 + 2n$

Where $n$ is the number of shaded discs. This indicates that the number of white discs increases by 2 for each additional shaded disc, starting with 3 white discs when there are no shaded discs.