At the standard (sea level) 761 miles per hour, sound travels about 12.68 miles per minute. So if it was heard 7 minutes later then it was about 88.78 miles away. Also, I'm sure he was approximating the time.
He would have felt the ground waves sooner.
you guys are mis-reading your word problem. The flash and smoke he sees are in the sky, the meteor hasn't hit yet. He goes about what he is doing, and hears the boom 7 minutes later. If the flash he saw was the meteor hitting the ground you could measure the speed of sound, but he sees it in the air, and it hits an undetermined time later. Thats the way I read it...