While measuring the length of a knitting needle, the reading of the scale at one end is 3.0 cm and at the other end is 33.1 cm. What is the length of the needle?

Given,
The reading of the ruler at one end, $l_1 = 3.0 cm$
The reading of the ruler at the other end, $l_2 = 33.1 cm$

The difference of the reading of the ruler, 

$l = l_2 - l_1$

$$\begin{gathered} l = 33.1 cm – 3.0 cm \\ l = 30.1 cm \end{gathered}$$

The length of the needle is $30.1 cm$.