2/5 of the fish in gary's tank are guppies. one fourth of the guppies are red. what fraction of the fish in gary's tank are red guppies? what fraction of the fish in gary's tank are not red guppies?

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

other answer is correct. i had used the radius of the radius somehow. a website: