The time between a lighting flash and the following thunderclap may be used to estimate, in kilometers, how far away a storm is. how far away is a storm if 12 seconds elapse between the lighting and the thunderclap
use the formula d = t/3 . where t is the time in seconds between the flash and the thunderclap
a 4 kilometers
b 5 kilometers
c 15 kilometers
d 36 kilometers

its 50 miles away from the starting point

we have three similar triangles, because each has a right angle and shares an angle.   let's write the angles in order: opposite to short leg, long leg, hypotenuse.

cab similar to bad similar to cbd

or as ratios,

ca: ab: cb = ba: ad: bd = cb: bd: cd

we also know

ac = ad + cd

(ad+cd): ab: cb = ba: ad: bd = cb: bd: cd

(ad+cd)/cb=cb/cd

we have cb=6, ad=5 and seek x=cd.

we reject the negative root and conclude x=4

answer: 4