We know that there are 20.0 fringes per centimeter, so we can find the distance between each fringe (fringe spacing) by dividing 1 cm by 20.0 fringes.
Fringe spacing = $\frac{1 \,\text{cm}}{20.0 \,\text{fringes}} = 0.05 \,\text{cm}$ per fringe
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