The mode is the number that occurs the most often in a list of numbers.
John's times: 648 596 575 628 605 609 565
No number appears more times than any other, so there is no mode for John's data.
Lukes times: 542 481 469 502 449 481 532
The number 481 appears the most times in the list, so the mode of Luke's data is 481.