Mary buys a 10-meter spool of thread for sewing. she uses 210 centimeters. how much is left on the spool in centimeters?

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

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.