# An SAT practice test question that needs explanation

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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