Answer :

The price of the chair afterwards was $176.40.  You take $210 and multiply it by .40 and get $84.  Then you subtract it from 210 and get 126.  Then take 126 multiply it by .40 and get $50.40 then you add it back to the $126 and get $176.40

The best way to find percent is to use proportion. After using proportion your answer would probably be 84$. Remember the percent always goes over 100. so u would put 40 over 100 and 210 under n which is the part you don't know.