Basically, deca- means ten of something, and deci- means one-tenth, so 1 meter = 10 decimeters, therefore 10 decameters = 10 x (10 decimeters) = 100 decimeters. Centi- means one-hundredth, so 1metre = 100 centimeters, therefore 1 decameter = 10 x (100 centimeters) = 1000 centimeters. Hecto- means one hundred, therefore 1 hectometer = 100 meters = 10 decameters. Kilo- means one thousand, of course, so 1 kilometer = 1000 meters, and since 1 hectometer = 100 meters, 1 kilometer = 10 hectometers. The only ones your daughter needs to bother about are millimeter, centimeter, and kilometer, and she should learn to manipulate these confidently. 1 centimeter = 10 millimetres 1 meter = 100 centimeters, or 1000 millimeters.