Two teachers led a number of students to travel (the tour fee was paid in a unified way), and they contacted two companies, and the quotations were each 100 yuan.

Solution: Let the total number of people be X, the charge of travel agency A is y 1, and the charge of travel agency B is y2.

Then y1= 2×100+(x-2 )×100× 75% = 75x+50.

y2=80%x× 100= 80x

When y 1 > y2, 75x+50 > 80x, the solution is: x < 10.

When y 1 = y2 and 75x+50 = 80x, the solution is: x= 10.

When y1< y2,75x+50 < 80x, the solution is: x > 10.

Therefore, when the number of people is less than 10, it is more affordable to choose B travel agency;

When the number of people is equal to 10, it is equally affordable to choose travel agencies A and B;

When the number of people is above 10, it is more affordable to choose a travel agency.