First of all, there are 3 100 men and women in a conference hall. If 25% of men leave the hall and 400 women come in, the number of men and women in the conference hall will be the same. How many women were there in the meeting room at first?

Answer ~ suppose there are x women at the beginning, then there are 3 100-X men at the beginning.

x+400 =(3 100-x)/( 1-25%)，

X= 1 100 people.