3. Teacher $D$ has 60 red glass beads and 50 black glass beads. A magical machine, when used once, will turn 4 red glass beads into 1 black glass bead, or turn 5 black glass beads into 2 red glass beads. After Teacher $D$ used the machine 30 times, all the red glass beads were gone. At this point, there are $\qquad$ black glass beads.
【Answer】 20