Before the Dessert Storm troopers arrived, a team of soldiers bent on demolishing a refinery smokestack in .?

Before the Dessert Storm troopers arrived, a team of soldiers bent on demolishing a refinery smokestack in Kuwait, set their mortar to 60-degree inclination and fired at the target 866 meters away horizontally. The smokestack was hit at the base on ground level but remained standing. So, after towing the carriage closer to the target, and at the same angle, they fired a second round, where seconds after, the smokestack began to topple away from the soldiers even as the projectile was yet on its way, still in the air. And at the instant the 100-m smokestack inclined at 45 degrees, it was hit on the upper tip. If the muzzle velocity remained constant at 100m/sec in both firings, how much closer was the mortar moved?