If Pete walks on hands at 1/4 what Tracy can do 1-handed cartwheels, then Tracy can do 1-handed cartwheels at 2*4=8 miles per hour.
Since Tracy does 1-handed cartwheels twice the rate of Susan's forward walking, then Susan walks at 8/2=<<8/2=4>>4 miles per hour.
Pete walks backwards 3x faster than Susan walks forward, and thus Pete walks backwards at 4*3=12 miles per hour.
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