Question:

The regular mathematics faculty could not teach because of being sick. As a stop-gap arrangement, different visiting faculty taught different topics on 4 different days in a week. The scheduled time for class was 7:00 am, with a maximum permissible delay of 20 minutes. The monsoon made the city bus schedules erratic, so the classes started at different times on different days.

Mr. Singh did not teach on Thursday. Calculus was taught in the class that started at 7:20 am. Mr. Chatterjee took the class on Wednesday, but he did not teach probability. The class on Monday started at 7:00 am, but Mr. Singh did not teach it. Mr. Dutta did not teach ratio and proportion. Mr. Banerjee, who did not teach set theory, taught a class that started five minutes later than the class in which probability was taught. The teacher in Friday's class taught set theory. Wednesday's class did not start at 7:10 am. No two classes started at the same time.

77. The class on Wednesday started at:

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Fix Mr. Singh to Friday (set theory) and Mr. Chatterjee to Wednesday first, then use the five-minutes-after-the-probability-class clue to place Mr. Dutta on Monday and Mr. Banerjee on Thursday.
Updated On: Jul 10, 2026
  • 7:05 am and the topic was ratio and proportion.
  • 7:20 am and the topic was calculus.
  • 7:00 am and the topic was calculus.
  • 7:20 am and the topic was calculus.
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The Correct Option is B

Solution and Explanation

Step 1: Fix the teacher who is not on Wednesday or Thursday.
Mr. Chatterjee is on Wednesday (given directly), so he is fixed there. Mr. Singh is not on Thursday and not on Monday (the clue says Mr. Singh did not teach the Monday class), so Mr. Singh must be on Wednesday or Friday. Since Wednesday already belongs to Mr. Chatterjee, Mr. Singh teaches on Friday.

Step 2: Use the Friday clue to fix Mr. Singh's topic.
The teacher in Friday's class taught set theory. Since Mr. Singh is on Friday, Mr. Singh taught set theory. This also fits the clue that Mr. Banerjee did not teach set theory, because Mr. Banerjee is a different person from Mr. Singh.

Step 3: Work out who taught probability.
Mr. Chatterjee (Wednesday) did not teach probability, and Mr. Singh (Friday) taught set theory, not probability. Mr. Banerjee cannot be the probability teacher either, because his class started five minutes after the class in which probability was taught, which means Mr. Banerjee and the probability teacher are two different people. That leaves only Mr. Dutta to teach probability, which fits the separate clue that Mr. Dutta did not teach ratio and proportion.

Step 4: Place Mr. Dutta and Mr. Banerjee on the remaining two days.
Monday and Thursday are left for Mr. Dutta and Mr. Banerjee. The Monday class started at 7:00 am. If Mr. Banerjee were on Monday, his start time would be 7:00 am, and the probability class, which starts five minutes before his class, would have to start at 6:55 am, before the earliest allowed time of 7:00 am. That is impossible, so Mr. Dutta is on Monday, teaching probability at 7:00 am, and Mr. Banerjee is on Thursday, at 7:00 am + 5 minutes = 7:05 am.

Step 5: Work out the topics left for Mr. Chatterjee and Mr. Banerjee.
Only ratio and proportion and calculus remain, for Mr. Chatterjee (Wednesday) and Mr. Banerjee (Thursday). Calculus was taught in the class that started at 7:20 am. Mr. Banerjee's class started at 7:05 am, so he did not teach calculus; he taught ratio and proportion. That leaves calculus for Mr. Chatterjee, so his Wednesday class started at 7:20 am, which also fits the clue that Wednesday's class did not start at 7:10 am.

Final Answer:
The full schedule is: Monday 7:00 am, Mr. Dutta, probability; Wednesday 7:20 am, Mr. Chatterjee, calculus; Thursday 7:05 am, Mr. Banerjee, ratio and proportion; Friday, Mr. Singh, set theory. So the class on Wednesday started at 7:20 am with calculus as the topic. In the original five-option paper this exact statement, 7:20 am and calculus, was printed twice, once as option B and once as option D, which made the question impossible to grade with a single unique correct choice. That duplication is why XAT officially dropped this question; on merit, the content shared by options B and D is the one that fits every clue. \[ \boxed{\text{7:20 am, calculus}} \]
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