Two temperature scales A and B are related by A-42100=B -72220.  At which temperature, two scales have the same readings?  [WB JEE 2011]

The relation between the temperature scales A and B is given as A-42100=B -72220, so for the two scales to have the same reading, A = B⇒                    A-42100=A -72220⇒                    A-421=A -722⇒                    2A-84=A-72⇒                      2A-A=84-72⇒                              A=12∘