The least count of a measuring instrument is the lowest amount that can be measured in it. Here is this problem, there are 5 marks between 10 mL to 20 mL i.e . 5 division between a gap of 10 mL . The value of each division is equal to 10 / 5 = 2mL which is the least we can measure in the cylinder .

Introduction:
A measuring cylinder is a common laboratory instrument used to measure the volume of liquids. It consists of a cylindrical tube with a narrow, graduated scale marked on its surface. The markings on the scale indicate different volume levels, allowing for accurate measurements.

Definition of Least Count:
The least count of a measuring instrument is the smallest value that can be measured using that instrument. It determines the precision or accuracy of the measurements. In the case of a measuring cylinder, the least count refers to the minimum volume difference that can be accurately measured.

Calculation of Least Count:
To calculate the least count of the measuring cylinder, we need to determine the volume difference between two adjacent markings on the scale. In this case, there are 5 marks between 10 mL and 20 mL.

Step 1: Calculate the total volume difference:
The total volume difference between 10 mL and 20 mL can be calculated by subtracting the lower value from the higher value:
Total volume difference = 20 mL - 10 mL = 10 mL

Step 2: Divide the total volume difference by the number of divisions:
To find the volume difference between two adjacent markings, we divide the total volume difference by the number of divisions (in this case, 5):
Volume difference between adjacent markings = Total volume difference / Number of divisions
Volume difference between adjacent markings = 10 mL / 5 = 2 mL

Step 3: Calculate the least count:
The least count is the volume difference between two adjacent markings:
Least count = Volume difference between adjacent markings
Least count = 2 mL

Explanation:
The least count of the measuring cylinder is 2 mL. This means that the smallest volume that can be accurately measured using this cylinder is 2 mL. Any volume measurement below this value will not be precise or reliable.

Conclusion:
The least count of a measuring cylinder with 5 marks between 10 mL and 20 mL is 2 mL. Understanding the least count is important for ensuring accurate and precise measurements in the laboratory. By knowing the least count of an instrument, scientists and researchers can determine the level of precision and reliability of their measurements.