100 cm = 1 meter
210 cm = 2.1 meters (multiply both sides by 2.1)
Mary uses 2.1 meters of thread so far, meaning she has 10-2.1 = 7.9 meters left
7.9 meters = (7.9 m)*(100 cm/1 m) = 790 cm
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Alternate route:
10 meters = (10 m)*(100 cm/1 m) = 1000 cm
Mary uses 210 cm out of 1000 cm total, so she has 1000-210 = 790 cm left.
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Answer: 790 cm