Let's define
x: time, in hours
Mime A charges 90 + 25x dollars
Mime B charges 45 + 40x dollars
If both mimes charge the same amount of money:
90 + 25x = 45 + 40x
90 - 45 = 40x - 25x
45 = 15x
45/15 = x
3 = x
The two mimes will charge the same amount of money after 3 hours.