An algebra teacher gave his students the following problem: "A car is travelling along a straight road at constant speed. The car passes a mile marker sign which is a two digit number "XY". One hour later it passes mile marker "YX". After one more hour it passes mile marker "X0Y" where the central digit is a zero. All numbers are in base 10".

One of the students received a paper that was smudged in one spot, making a single character of the above quoted instructions look like some other character.

When everyone in class shared their answers, this student exclaimed: "Hey, compared to the answers you all got, my mile marker signs all look upside down, and the speed I got was 8/3 times too fast!"

Part 1. List all three signs and the speed of the car in miles per hour.

Part 2. What character was changed, and what was it changed to?