Answer
1400 lines/cm
Work Step by Step
The number of slits per cm is the reciprocal of the slit width (measured in cm).
Use equation 24–4 to find 1/d.
$$d sin \theta = m \lambda $$
$$\frac{1}{d}=\frac{ sin \theta}{ m \lambda }$$
$$=\frac{ sin 15.0^{\circ}}{3(6.20\times10^{-5}cm)}$$
$$=\frac{1392}{cm}= \frac{1400}{cm}$$
