It takes 6.5 seconds for a train to pass a sign, 23 seconds to pass a 300 meter long bridge, and a few seconds to cross an 800 meter long cave

It takes 6.5 seconds for a train to pass a sign, 23 seconds to pass a 300 meter long bridge, and a few seconds to cross an 800 meter long cave

The answer is 50.5 seconds
Passing the bridge is equivalent to 300 meters more than passing the road sign, 23 seconds more - 6.5 seconds = 16.5 seconds, and the speed is 300 / 16.5 meters / second
It's equivalent to walking 800 meters more through the cave than through the road sign. You should walk 800 * 16.5/300 = 44 seconds more, 44 + 6.5 = 50.5 seconds in total