Cynthia's afternoon train normally arrives at her station at 5:30 pm each day, where she is picked up by Alan and driven home. One day she was on an earlier train which arrived at 5 pm, and she decided to walk in the direction Alan was coming from home. Alan had left in time to meet the 5:30 pm train, but this time he picked up Cynthia and they arrived home 10 minutes earlier than usual. Suppose the speed of Alan's car is 30 miles per hour, what is the walking speed Cynthia in miles per hour