Question

Find the greatest possible lenght to measure 495 cm, 900cm,

Answer 1

To find the greatest possible length to measure 495 cm and 900 cm, we have to find the HCF or Highest Common Factor of 495 and 900. 

FINDING THE HCF
You can use any method to find HCF, but now I am going to use the long division method to find HCF. 
We have to find the HCF of 495 and 900. 

                              
              495 | 900 | 1
                     - 495          
                       405 | 495 | 1
                              - 405          
                                  90 | 405 | 4
                                       - 360        
                           HCF ⇒    45 | 90 | 2
                                                - 90
                                                    0

As you can see, when we divided 90 by 45, the remainder came to be zero. So, the HCF is 45. 

Therefore, 45 cm is the greatest length which can be used to measure 495 cm and 900 cm. 

Hope this may help you. 

If you have any doubt, then you can ask me in the comments.