There is a delay of 5 seconds between the time you see a flash lightning and hear the thunder. if the temperat

Question

ure is 20 C , how far away is the lightning strike?

Answer 1

May I presume this is for a class assignment?  If so, you will no doubt want to be more precise as follows:

With V being the speed of sound in meters per second,

And Tc being the temperature in degrees Celcius

The formula is:

V ≈ 331.4 + 0.6Tc

so you get 331.4 + 0.6 x 20

which gives you 331.4 + 12.0 = 343.4 meters per second

multiplying by five seconds, you get

343.4 meters/second x 5 second = 1717 meters

Answer 2

The temperature doesn't matter much. The speed of sound in air is generally somewhere in the range of 340 ± 10 m/s, and let's say that your observation has an error bar of ± 0.5 s or so.

Multiplying these numbers out (5 s * 340 m/s), you'll get a distance of 1.7 ± 0.2 km.