Bob has a coin cup with four $1 tokens and two $5 tokens in it. He also has two $10 tokens and one $25 token in his pocket. He randomly draws a token from the cup, and then randomly draws a token from his pocket. What is the probability that he
will draw $30 in tokens?

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