A monochromatic light of wavelength 6000 o A is incident on the...

The linear width of central maxima is given by, 

$W = \dfrac{2\lambdaD}{d} = \dfrac{2 \times 6 \times {10}^{- 7} \times 0 .2}{1 \times {10}^{- 5}}$

$= 2 .4 \times {10}^{- 2} = 24mm$