900 x 3 / 4 = 675 people came before the start of the concert. 900 - 675 = 225 people did not come before the start of the concert. 225 x 5 / 9 = 125 people came few minutes after the first song. So, 125 + 80 = 205 people came to the concert that was not able to be at the start of the concert. Therefore, 225 - 205 = 20 people did not go to the concert.
This justifies what answer for what question? Q & A:
Nine hundred tickets were sold to a concert. Three-fourths of those who bought the ticket came before the start of the concert. Five-ninths of the remaining came few minutes after the first song. Eighty people arrived during the middle part of the concert while the rest did not go. How many of those who bought the tickets did not go?
20