What is the ratio of 25 cm to 9 dm?

The centi- prefix means one hundredth → 1 cm = 0.01 cm

The deci- prefix means one tenth → 1 dm = 0.1 cm

→ 25 cm : 9 dm

= 25 × 0.01 m : 9 × 0.1 m

= 0.25 m : 0.9 m

= 0.25 : 0.9

= 25 : 90

The radio of 25cm to 9 dm is 25:90