Answer: 8.4.
Here is a trick to know how to solve stuff like this:
K H D M D C M<--------(You could also remember it by "Kangaroos Hopping Down Mountains Drinking Chocolate Milk).
Here's what they stand for:
Kilo Hecta Deca Meters, Liters, or Grams Deci Centi Milli
Now, you do this:
K H D M D C M
84
You put 84 under "Milli", since it's 84 millimeters . Now, you go to da LEFT, and you move da decimal to da LEFT. (Every whole umber automatically gets a decimal at da end of it. Example: 72 is da same as 72.) (Decimal is at da end of 72). So now, 84 is under centi, (in this case, centimeters), and you move da decimal ONCE, since you moved from milli to centi ONCE. Now it would be 8.4. Since da problem only asked for centimeters, YOU'RE DONE! :-).
