Xiaoming and Xiaoli compete to climb stairs. When Xiaoming runs to the 6th floor, Xiaoli just runs to the 4th floor. At this speed, when Xiaoming reaches the 21st floor, how many floors can Xiaoli run to?

（21-1）×（4-1）/（6-1）+1
=20×3/5+1
=12+1
=13th floor
Xiao Li can run to the 13th floor

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