To solve this question, you would need to uncover the equation behind that pattern.
1 -> 3 -> 6 -> 10
In this case, this is a triangular pattern which the equation for the amount of dots in a triangle is:
n(n+1) / 2
n represents the pattern no.
Let’s test this out
1(1+1) / 2 = 1(2) / 2 = 2 / 2 = 1
2(2+1) / 2 = 2(3) / 2 = 6 / 2 = 3
3(3+1) / 2 = 3(4) / 2 = 12 / 2 = 6
4(4+1) / 2 = 4(5) / 2 = 20 / 2 = 10
As you can see, this equation holds up.
To find r, you would need to make another equation with info provided from the question as such:
r + 3 sequence - r sequence = 126
To translate it into a math equation you do as such, since you know the equation, you can easily replace n with r or r + 3, so:
(I shown in pic)