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I am not sure if there is a rule or something that I'm missing but I just noticed that the sides of the triangle 15,20,25 are applicable to Pythagoras' theorem c^2=a^2+b^2.
25^2=20^2+15^2
625=400+225

This means the triangle has a right angle between the sides 15 and 20. Calculate surface area 15*20/2 = 150.
From this we can calculate half of the common chord which would effectively be the height of the triangle to the hypotenuse.
(height*25)/2 = 150
height = 12
common chord = height x 2 = 24 i.e. Answer choice E

I tried transposing/rotating/attempting to calculate different proportions of angles/sides but the results were inconclusive. After some time I noticed that 15-20-25 is a right angle triangle.

If anyone has a more detailed/less lucky explanation, please give your two cents.
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