The way I would look at it is that Sophia types 15 words per minute faster than Patty so every minute Sophia is 15 minutes closer to what Patty has done. 510/15=34 so after 34 minutes Sophia will have caught up to Patty
The mean of a data set is the average, and can calculated by adding up all the values and then dividing the sum by the number of values in the data set.