Question 1131556
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His two test scores so far are 87 and 91, a total of 178.<br>
To get an average of 90 or more on three tests, the sum of the three test scores must be 270 or more.<br>270-178 = 92; he needs at least a 92 on the third test to average 90 or more and get an A.<br>
Here is a faster and easier way to answer a question like this involving an average of numbers that are all close together: compare each number to the desired average to see where the total lies compared to the average.<br>
In this problem, the desired average is 90.  The first score was 87, which is -3 compared to the average; the second score was 91, which is +1 compared to the average.  So the first two tests combined are -2 compare to the average.<br>
That means the third score, in order to attain the desired average, must be +2 (at least) compared to the average:  90+2 = 92.