Three-fourths or 3/4 of the seats are taken out of the 500 would be 3/4 = 0.75 or 75% of the seats taken out of the 500 seats in total.
500 / 75% or 500 * 0.75 = 375
So, your answer would be that 75% of the seats were taken out of the total of 500 seats. Which would be 375 seats were taken out of 500 seats in total.
Hope this helps.
~ ShadowXReaper069