Time for Xiao Hong to go from point B to A: 48 (Hong minutes) - 48 (Ming minutes)
You need to convert (Ming minutes) to (Hong minutes); the ratio is found by noting that Xiao Ming gets to run a shorter time of 48-20=28 (Ming minutes), when Xiao Hong runs 10 (Hong minutes) ending up with the the conversion factor of 10/28.
48-(48*10/28)
30.85714286 minutes for Xiao Hong to run from Point B to A.