Answer
(a) The number of fringes would stay the same.
(b) The fringe spacing would stay the same.
(c) The width of each fringe would stay the same.
(d) The brightness of each fringe would increase.
Work Step by Step
We can write a general equation for the fringe spacing $y$:
$y = \frac{\lambda~L}{d}$
(a) The fringe spacing does not depend on the number of slits. Therefore, the number of fringes would stay the same.
(b) The fringe spacing does not depend on the number of slits. Therefore, the fringe spacing would stay the same.
(c) The width of each fringe does not depend on the number of slits. Therefore, the width of each fringe would stay the same.
(d) Since more light would get through 20 slits, the brightness of each fringe would increase.
