What is 0.55 hectometers expressed in decimeters?

Your answer is very simple. It is 550 decimeaters.

A Hectometre (1 Hm) is 100 m so 0.55 Hm = 55 m; 1 decimetre = 0.1 m so 55 m = 55 x 10 dm = 550 dm

The conversion from hectometers to decimeters may be done by proper dimensional analysis and the conversion factors. Each hectometer is composed of 100 meters and each meter is composed of 10 decimeter. From this,

(0.55 Hm) x (100 m / 1 Hm) x (10 dm / 1 m) = 550 decimeter

Thus, 0.55 hectometer is equal to 550 decimeters.