Help with a ukcat question?

Marlow, Abdul, Kim, and John all had a study session scheduled for 1pm. Abdul came 10 minutes after Marlow, who came 5 minutes before John. John came 15 minutes late to the meeting. Kim came before Abdul, but after the study session was due to start.

Which of the following statements MUST be true?

A
All four arrived within 15 minutes of each other
You incorrectly answered: A
16.23% of users chose this answer
B
All four arrived within 20 minutes of each other
Correct Answer
47.40% of users chose this answer
C
Kim came first to the study session
16.88% of users chose this answer
D
John came second to the study session
19.48% of users chose this answer
Answer Explanation
Marlow 1:10
Abdul 1:20
Kim 1-1:20
John 1:15




A quick table will help. John is the anchor, as we know he arrives at 1.15pm (15 minutes later than planned).

The answer that must be true is that all four arrived within 20 minutes of each other. There is no way to tell if Kim came first or not, or whether John came second as we don’t have Kim’s exact time of arrival.

I don't understand the latest someone can arrive is 1:20 and earliest 1:10 therefore the smallest window must be 10 mins so shouldn't 15 mins also be correct? Why is 20 mins favored?

Marlow, Abdul, Kim, and John all had a study session scheduled for 1pm. Abdul came 10 minutes after Marlow, who came 5 minutes before John. John came 15 minutes late to the meeting. Kim came before Abdul, but after the study session was due to start.

Which of the following statements MUST be true?

A
All four arrived within 15 minutes of each other
You incorrectly answered: A
16.23% of users chose this answer
B
All four arrived within 20 minutes of each other
Correct Answer
47.40% of users chose this answer
C
Kim came first to the study session
16.88% of users chose this answer
D
John came second to the study session
19.48% of users chose this answer
Answer Explanation
Marlow 1:10
Abdul 1:20
Kim 1-1:20
John 1:15




A quick table will help. John is the anchor, as we know he arrives at 1.15pm (15 minutes later than planned).

The answer that must be true is that all four arrived within 20 minutes of each other. There is no way to tell if Kim came first or not, or whether John came second as we don’t have Kim’s exact time of arrival.

I don't understand the latest someone can arrive is 1:20 and earliest 1:10 therefore the smallest window must be 10 mins so shouldn't 15 mins also be correct? Why is 20 mins favored?

John 1.15pm as you said so Marlow is 1.10 [5 mins earlier].

Then Abdul is 1.20 [10 mins after Marlow].

Kim arrives late ie after 1pm but before Abdul so before 1.20pm

The solution is literally in front of you. The latest someone can arrive is 1:20, as you said. Kim came at an unspecified time after the study session began, but before Abdul. Can you see that if Kim arrived at 1:00(+1second) that there would be a 20 minute gap between her and Abdul?