DAY 78

Comment

One approach is to imagine cutting the line into 1 cm lengths, and placing the lengths next to each other to make a thicker line:
for example, two lengths make a 1 cm line 0.4 mm thick, 3 lengths make a 1 cm line 0.6 mm thick, and so on.

Answer

The line would have to be 50 cm long.

Readers' methods

R Orlenkov, Minsk:
0.2 mm is 0.02 cm.
0.02 cm x 50 cm = 1 square cm.

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