pikachu wrote:

Fifty percent of all the students attending a school on a certain day arrived by 7:00 AM. How many students arrived by 7:00 AM on that day?

(1) Fifteen students arrived between 7:00 AM and 8:00 AM, and 4/5 of that day’s total attending students arrived by 8:00 AM.

(2) Ten students arrived after 8:00 AM that day.

Can anybody help me understand the question stem. The question is asking how many students arrived by 7:00am on that day but the first part of the question already says 50% of all the students arrived by 7am, so isn't the first part already answering the question itself. Seem confusing.

You need to have a conclusive value for the number of students arriving by 7 am on that day. From the question stem, all we know is that 50% of the total students in attendance arrived by 7am.

From F.S.1, we know that 15 students arrived between 7 and 8 am. Also, by 8 am, total of 4/5 students had arrived. Thus, let the total number of students be 10x. Thus,

8x = 15+(the number of students who arrived by 7am) = 15+5x.

We can solve for x and get the value. Thus sufficient.

F.S 2 only states that 10 students arrived after 8 am. It tells nothing about the total number of students. There might be more students coming in. Thus insufficient.

A.

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