One of the 'keys' to this question (and all DS questions) is to be a bit cynical about the information that you've been given. Think about what you KNOW and what you DON'T KNOW and be prepared to do the necessary work to find any patterns that exist in the information.
In this prompt, we're asked for the number of hours that Suresh traveled yesterday. We're given no information to start with, but we DO know that he could not have traveled for more than 24 hours (because there's only 24 hours in a day).
Fact 1: Yesterday, Suresh traveled from 1 A.M. to 1P.M. without a break.
This tells us that Suresh traveled for AT LEAST 12 hours. He COULD have traveled for some additional time before 1am or after 1pm, so we don't know how many hours he traveled on that day in total.
Fact 1 is INSUFFICIENT
Fact 2: Yesterday, Suresh traveled half the total number of hours he traveled the day before yesterday.
This doesn't give us any direct numbers to work with, but it DOES establish a ratio. The time spent traveling yesterday was HALF what it was the day before yesterday. This means that Suresh could have traveled AT MOST 12 hours (since there are only 24 hours in a day, and HALF of that is 12 hours), but we still don't know the exact number of hours traveled.
Fact 2 is INSUFFICIENT
Combined, we know.....
Suresh traveled AT LEAST 12 hours
Suresh traveled AT MOST 12 hours
These Facts tell us that Suresh must have traveled exactly 12 hours.
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