What is the highest numbered square that is painted blue on a 1x100 square board?

Question

On a board measuring 1x100, each square is numbered from 1 to 100.
Three colors are used to paint the squares from left to right. The following pattern is repeated: one blue square, two red squares, three green squares, one blue, two red, three green and so on.
What is the highest numbered square that is painted blue?

Answer 1

It forms an AP
a = 1
d = 5

now by guessing the value of n and finding the one closest to 100 is the answer

1st try = 1 +20*5 = 101 not the answer
2nd try = 1+19*5 = 96

so n-1 = 19
n =20
answer is 20 blocks