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 12 ​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=12−​(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​?