In a young's double slit experiment, the difference in optical path length between the rays starting from the slits `S_1` and `S_2` and reaching a point A, on the screen in 0.0075mm and reaching another point B on the screen, on the other side of the central fringes is 0.0015 mm. How many bright and dark fringes are observed between A and B if the wavelength of light used is `6000 A^@`