13. Water is spilling out of a tank with a leal-t at the bottom. Attert hours. there are t + E liters that had spilled out- At what rate is the water spilling out at t = 9 minutes? 14. A balloon with a shape of a sphere is tted with air at a rate of 1 cubic meter per minute- Compute the time rate of change of its surface area if it is already lled with 36:: cubic meters ol' air. 15. Determine the we: of erasing = x y