An SAT practice test question that needs explanation

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4. Mai 2017 14:46

So on the third SAT practice test in the math section that allowed calculator, this problem stumped me. The correct answer was 50, but no explanation was given. Any help?

An online store receives customer satisfaction ratings between 0 and 100, inclusive. In the first 10 ratings the store received, the average (arithmetic mean) of the ratings was 75. What is the least value the store can receive for the 11th rating and still be able to have an average of at least 85 for the first 20 ratings?

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Inger Hohler

5. Mai 2017 07:27
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If the average rating for 20 ratings is at least 85, it means your total has to be least 1700 points. You are looking for the minimum value for the 11th rating, so the sum you are interested in is exactly 1700.

You are trying to find the lowest number of points for the 11th rating. The lowest possible value for the 11th rating is what you have if all the 9 subsequent ratings were at 100 points.

What is the maximum number of points that can be delivered in the last 9 ratings?

How many points were delivered in the first 10 ratings?

I hope that this puts you on the right track!
vivien cheng

20. April 2018 21:39
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85 x 20 = 1700 (average of 85 with 20 ratings)

75 x 10 = 750 (already have 10 ratings of 75)

100 x 9 = 900 (9 remaining ratings)

1700 - 750 - 900 = 50

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An SAT practice test question that needs explanation

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