After 3 red buttons on a pattern, 4 black buttons are placed. After using 15 red buttons. how many black buttons are used?
Think it again.
3 red, 4 black, 3 red, 4 black, 3 red, 4 black, 3 red, 4 black, 3 red, 4 black.
3 red, 4 black, 3 red, 4 black, 3 red, 4 black, 3 red, 4 black, 3 red, 4 black.