Physics for Scientists and Engineers: A Strategic Approach with Modern Physics (4th Edition)

$I_1 = 3~mW/m^2$
We can find the light intensity when one slit is covered: $I_{max} = 4~I_1$ $I_1 = \frac{I_{max}}{4}$ $I_1 = \frac{12~mW/m^2}{4}$ $I_1 = 3~mW/m^2$