While measuring the length of a knitting needle with a ruler, the reading of the ruler at one end is 3.0 and the other end is 33.1cm. What is the length of the needle?
$
  (a){\text{ 33}}{\text{.1cm}} \\
  (b){\text{ 3}}{\text{.0cm}} \\
  (c){\text{ 30}}{\text{.1cm}} \\
  (d){\text{ 36}}{\text{.1cm}} \\
 $

Hint: In this question use the concept that the reading goes on increasing as we go from left to right thus the length of the knitting needle will actually be the difference of the reading of the ruler at the other end and the reading of the ruler at the one end. This will help approach the solution of this problem.

Complete step-by-step answer:
Given data:
The reading of the ruler at one end = 3.0 cm
And the reading of the ruler at the other end = 33.1 cm
While measuring the length of the knitting needle with a ruler the actual reading of the knitting needle is the difference of the final reading and initial reading.
I.e. the difference of the reading of the ruler at the other end and the reading of the ruler at the one end.
So the actual reading of the knitting needle or the length of the needle = reading of the ruler at the other end – reading of the ruler at one end.
Now substitute the values we have,
So the actual reading of the knitting needle or the length of the needle = 33.1 cm – 3.0 cm = 30.1 cm.
So the length of the needle is 30.1 cm.
So this is the required answer.
Hence option (C) is the correct answer.

Note – In this question we have taken the difference between the final reading and the initial reading only because the final reading was large in comparison to the initial reading but however, if we would have taken the difference between the initial reading and the final reading then the modulus of the final output obtained need to be taken as length can never be negative.