Un arquitecto ha pagado en un corralón un total de $ 130 por 20 Ladrillos, 5 kg de Arena y 10 kg de Cemento. Calcular el precio de cada articulo, sabiendo que 1 kg de Cemento cuesta el triple que 1 Ladrillo y que 1 kg de Arena cuesta igual que 4 kg de Cemento más 4 Ladrillos. Siendo: número de ladrillos y: kg de arena kg de cemento

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