maths
posted by jasmine .
the mean of a set of test scores is 64. a new student takes the same test and scores 80 marks. when his score is added to the other scores, the mean increases to 65. how many students sat for the test altogether?

Say there are n scores. If the mean is 64, then the total points is just 64n.
Now, we have one more score, and the mean rises to 65. So, we have n+1 scores with mean 65. And we have added 80 points to the total.
64n+80 = 65(n+1)
64n+80 = 65n+65
n = 15
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