S yin: S ladder = 4: (9-4) = 4: 5.

Let BC be x cm long.

( 15x)×( 1/2)×( 1/4)=[( 12-x)+ 12]× 15×( 1/2)×( 1/5)

x=32/3

The answer BC is 32/3 cm long.

There should be no problem. If there is any problem, correct it immediately.