Explorers on a small airless planet used a spring gun to launch a ball bearing vertically upward from the surface at a launch velocity of 15 m >sec. Because the acceleration of gravity at the planet’s surface was gs m>sec2, the explorers expected the ball bearing to reach a height of s = 15t - (1>2)gst2 m t sec later. The ball bearing reached its maximum height 20 sec after being launched. What was the value of gs?