ysabelcruz2003 wrote::cry:
Take it back to the basics!
Deci is the prefix for 1/10th, or 0.1.  one decimeter is one tenth of a meter.  In other words there are 10 decimeters per meter.
If there are 10 decimeters in a meter, them there are 100 square decimeters in a square meter:
(1 m)x(1m) =  1 m² 
(10 dm)x(10 dm) = 100 dm²