Hello

They must be equal so you can solve for x and find how many minutes the tank took to drain. They are equal because the volume of water in the tank when it is full is the same as the volume of water that drains out of it to make it empty.

The LHS is the amount of water in the tank x minutes after 10:00 am and the RHS is the amount that drains out of the tank in (68-x) minutes.