Flash and thunder are produced simultaneously. But thunder is heard a few seconds after the flash is seen, why?

Two open pipes of different lengths are producing their fundamental tones simultaneously and the number of beats per second is 5.if the shorter pipe has fundamental frequency 250 Hz,what is the length of the longer pipe?(velocity of sound = 340m/s)

When two tuning forks when sounded together,20 beats were produced in 8 seconds .After loading one of the forks with a littile wax,they produce 32 beats in 8 seconds.If the unloaded fork had a frequency of512 Hz,calculate the frequency of the other.