How do you convert 42cm to mm?

Hint: Check-out the different units of measurement of distance and the relation between them. Observe the relationship of conversion of units from centimeters to millimeters. Use the conversion: $- 1cm = 10mm$ to calculate the correct answer.

Complete step by step answer:
Here, we are provided with a length of 42cm and we have to convert it into mm, that is from centimeter to millimeter. First, let us see the definition of length or distance and the units needed to measure it.
Distance is a numerical measurement of how far apart objects or points are. In physics length or an estimation based on other criteria. Various fields have various formulas for measuring the distance between two points, however, the units used to represent them are the same.
Now, there are various units to represent the distance between two points that depends upon the extent of separation of the points. For example: - small distances are measured in units like millimeters, centimeters, feet, decimeters, etc. And large distances are measured in units like hectometers, kilometers, miles, etc. Here our main concern is to find the relationship between centimeters and millimeters. So, the conversion formula is given as - 1cm = 10mm, that is one centimeter’s measures ten millimeters.
Now, let us come to the question. We have been provided 42cm. So, we have,
$1cm = 10mm$
Multiply both sides by 42,
$ \Rightarrow 42 \times 1cm = 42 \times 10mm$
Simplify the terms,
$42cm = 420mm$

Note: One must remember the relationships between several units of distance measurement because there will be many questions in which we will be required for the conversion. Remember the conversion of basic units like a millimeter, centimeter, meter, and kilometer. You may apply a unitary method for getting the answer just like we did above but remembering the formulas of conversion will save our time in solving such questions.