What is the number 0.151515 a rational or irrational?

Whether it terminates at precisely 0.151515 or continues to repeat that same pattern forever, it is a rational number. To represent an infinitely repeating pattern, draw a horizontal line over the repeating numbers (in this case, over everything after "0."). If that isn't possible (because you're using a computer, like you are now), place an ellipsis (three periods, "...") after the decimal repeats a few times ("0.151515...").

Numbers that continue infinitely but never show an infinitely-repeating pattern are irrational.