Hi,

One of the few conversion factors I remember is that there are 2.54 centimeters in one inch. "Cent" is a prefix meaning ¹ /₁₀₀ (there are 100 cents in a dollar) and "milli" is a prefix meaning ¹ /₁₀₀₀ . Thus it takes 10 millimeters to make a centimeter and hence

30 millimeters is 3 centimeters and 20 millimeters is 2 centimeters.

Since there are 2.54 centimeters in an inch, 3 centimeters is a little more that an inch and 2 centimeters is a little less than an inch. To be precise

3 cm = ³ /_2.54 = 1.18 inches and
2 cm = ² /_2.54 = 0.79 inches.

Penny