The only one which is used in scientific terminology is the kilometer, although centimeters are used colloquially and quite acceptable, but the others should have fallen out of use decades ago (indeed, have done so) so exercises of this type are just a waste of time in my opinion.

Basically, deca- means ten of something, and deci- means one-tenth, so ONE meter = TEN decimeters, therefore TEN decameters = TEN x ( TEN decimeters) = HUNDRED decimeters.

Centi- means one-hundredth, so ONE metre = HUNDRED centimeters, therefore ONE decameter = TEN x ( HUNDRED centimeters) = ONE THOUSAND centimeters.

Hecto- means one hundred, therefore ONE hectometer = HUNDRED meters = TEN decameters.

Kilo- means one thousand, of course, so ONE kilometer = ONE THOUSAND meters, and since ONE hectometer = HUNDRED meters,
ONE kilometer = TEN hectometers. ...more

1hm = 100 metres = 100 x 100 cm = 10,000 cm

Is one/ten thousand