Question 848676
On the left side, the "80+10" will turn into 90 after you add 10 to 80.


So 


80+10=90-______


turns into


90=90-______


So a "0" must go in the blank because of the left side. If we use any other number, then 90 - "some number that's not 0" would change to something that's not 90.


So...


90 = 90-___<u><font size=4 color="red">0</font></u>___


and 


80+10 = 90-___<u><font size=4 color="red">0</font></u>___