Christina uses a ruler to measure the length of the math book she says that the book is 4/10 m long is her measurement in simplest form if not what is the length of the book in simplest form

  2/5 m or 4 dm

Step-by-step explanation:

The fraction 4/10 can be reduced by removing a factor of 2 from numerator and denominator. The length could be expressed as 2/5 m.

The metric system has unit prefixes that make it easy to choose appropriate units for almost any measurement. Apparently Christina wants to use 1/10 m as her unit of measurement. That length is also called a "decimeter." So, Christina's measurement could be expressed in very simple form as 4 dm.

A decimeter is not a particularly common unit in everyday use, though it is reasonably useful for many purposes.* More conventionally, lengths in that range are expressed in centimeters, so Christina's measurement could also be described as 40 cm.

* I like units of decimeters for volume calculations, because a cubic decimeter is 1 liter.