On using red light $(\lambda=6600 \mathop {\rm{A}}\limits^{\rm{o}})$ in Young's double slit experiment, 60 fringes are observed in the field of view. If violet light ( $\lambda=4400 \mathop {\rm{A}}\limits^{\rm{o}}$ ) is used, the number of fringes observed will be

Young's double slit experiment is carried out by using green, red and blue light, one colour at a time. The fringe width recorded are $\beta_{G^{\prime}} \beta_{R^{\prime}} \beta_B$ respectively, then