



 
Hi Florian, Suppose the total of the grades of all 30 students was $T$ then the overall average would be $\large \frac{T}{30}.$ But you know the overall average was 80. Solve for $T.$ Suppose $n$ students passed the test and the total of their grades was $t.$ You know the average grade of the students who passed the test was 84. This gives you an equation in $n$ and $t.$ Since $n$ students passed the test, the number who didn't pass the test was $30  n.$ The total of the grades of the students who didn't pass the test was $T  t.$ Their average was 60. This gives you a second equation in $n$ and $t.$ Solve for $n.$ Penny  


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