Suppose your friend selects one of the coins uniformly at random. Your friend flips the coin 10 times and tells you that it came up H 5 times and T 5 times. Based on this information, what is the probability that your friend selected c5

i believe the answer is the polygon, heptagon.

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