In a Young's double slit experiment, the slits are separated by 0.2 mm. If the slits separation is increased to 0.4 mm, …

The angular fringe width is given by the relation.
\(\beta =\frac{D\lambda }{d}\propto \frac{1}{d}\)
Hence the angular fringe width is inversely proportional to slit separation.
If the slit separation is doubled, the angular fringe width becomes half.
Therefore there is a 50 percent decrease in angular fringe width.