Find the difference between these measurements.
 $ 7L\,135mL $ and $ 12L\,4mL $

Hint: All the quantities are measured using standard units. These units represent a certain amount of the quantity. For example, 10 metres means 10 times the definite predetermined length that is represented as a metre. Units of volume are Litre, Millilitre etc. Litre is used to measure larger quantities of volume while millilitre measures smaller quantities, all the units are linked to each other by some power of 10. Litre is 1000 times the millilitre. Using this relation, you can convert the two measurements to the same units and thus find the difference.

Complete step-by-step answer:
We know that
 $
\Rightarrow 1L = 1000mL \\
  or\,1mL = \dfrac{1}{{1000}}L \;
  $
So,
 $ \Rightarrow 135mL = 135 \times \dfrac{1}{{1000}}L = 0.135L $
Thus, $ 7L\,135mL = 7.135L $
And
 $ \Rightarrow 4mL = 4 \times \dfrac{1}{{1000}}L = 0.004L $
So, $ 12L\,4mL = 12.004L $
The difference between the above two measurements is $ = 12.004 - 7.135 = 4.869L $
 $ 4.869L = 4L + 0.869L $
Now,
 $
\Rightarrow 1L = 1000mL \\
\Rightarrow 0.869L = 0.869 \times 1000 = 869mL \;
  $
So the difference between $ 7L\,135mL $ and $ 12L\,4mL $ is $ 4L\,869mL $ .
So, the correct answer is “,$ 4L\,869mL $”.

Note: Calculating the size or amount or degree of something by using an instrument or device is called measuring and the number obtained after measuring is called the measurement.
For example, we measure the mass of objects using a weighing machine.
There are some standard units assigned by the international system of units, these units are called S.I. units. These units are assigned to seven base quantities; all other quantities can be derived from these base quantities. S.I. The unit of mass is the kilogram, the length is measured in metre, second is the unit used to measure time, the kelvin for temperature, the ampere for electric current, the mole is used to measure the amount of substance, the candela for luminous intensity.