Weather: how far away was the lightning strike?

Self-tutoring about weather events: the tutor mentions a way to estimate how distant a lightning strike was.

Last evening, there was intermittent lightning and thunder. At one point I thought it was over, but then a flash reached the corner of my eye. I wasn’t looking outside, but was in a dark room, so noticed it through the blind.

“Was that lightning?” I asked myself. I started counting while I awaited thunder, in case it was. Around 20 seconds later, thunder finally arrived: indeed, it had been lightning.

Sound travels around 340m/s, so 20*340 = 6800m or 6.8km away.