In a situation like this I would say that she walked ~5m/s. Do you have walker 2 and 3 reversed in the first question though? its hard to see.
Your approach to the question is correct. The average speed can be found by dividing the distance traveled by the time.
In this case it looks like you just have to estimate the time to 12 or 12.5 seconds.
From your graph I read the time elapsed as being close to 12.5s
Average speed = 60/12.5 = 4.8m/s which is close to your second answer