Wave Optics

In a Fraunhofer diffraction of a single slit, when a slit is illuminated by a light of wavelength $6480\ \text{Å}$, angular width of central maximum is measured. When the slit is illuminated by light of another wavelength '$\lambda$' the angular width decreases by $25\%$. The value of $\lambda$ in $\text{Å}$ units is