Lights of wavelengthλ1=4500 A0 , λ2=6000 A0  are sent through a double–slit arrangement simultaneously. Then

Wavelength λ1=4500 A0 Wavelength λ2=6000 A0 λ1λ2=45006000=34 ⇒  4λ1=3λ2 i.e. the fourth bright fringe of λ1  coincides with the third bright fringe of λ2 .