What is the length of the gray object assuming the other end of it starts at zero. Make your measurement as exact as possible.
In your guess for its length, the 10's place is known with certainty. In other words it is at least 20 units long. The number in the 1's place is now known. The object is at least 9 units longer than 20. The next number is in the measurement is between the ruler's markings and is an estimated measurement. Next look at the measurement with a ruler more precise than this one.
by Tony Wayne ...(If you are a teacher, please feel free to use these resources in your teaching.)