2 answers
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Use a proportion, with the map scale as one fraction, and the other fraction having the unknown quantity (which is the actual distance).
We will represent the unknown quantity using the variable "d" but you can use "x" as well.
Proportion:
1/20 = 3.5/x
Solve using cross-products:
1x = 20 * 3.5
x = 70
Or the shortcut:
x = 20 * 3.5 / 1 = 70
Hope this helps!