What if a problem involves more than one unit for a characteristic(such as length)how can u tell which unit is more appropriate to report the answer in?

Larger units are more appropriate for describing larger things.

Like - To know the distance between two cities, we use miles or kilometers. To know the distance between two rooms of a house, we use feet.

Similarly, to know the weight of a person, we can use pounds or kilograms. But to get the weight of an airplane, we will use metric tons.

This is why it is important to use appropriate units. Shorter units will be inconvenient to use for large scale. Like you cannot tell the weight of an airplane in milligrams.

Similarly, you cannot tell the height of a person in kilometer or yards. It can be appropriate only when we use feet or centimeter for someone's height.