After they each performed, both were deemed equal until Apollo decreed they play and sing at the same time. As Apollo played the lyre, this was easy to do. Marsyas could not do this, as he only knew how to use the flute and could not sing at the same time. Apollo was declared the winner because of this. Apollo flayed Marsyas alive in a cave near Celaenae in Phrygia for his hubris to challenge a god. He then nailed Marsyas' shaggy skin to a nearby pine-tree. Marsyas' blood turned into the river Marsyas.
Who could not sing at the same time he played the flute?
Marsyas