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Using the mean concept, it is found that we will need 15 more points to bring up his average up to 90%.
The mean of a data-set is the sum of all observations divided by the number of observations.
In this problem:
Hence, the mean is:
[tex]M = \frac{0.84(5) + n}{5 + n} = \frac{4.2 + n}{5 + n}[/tex]
We want the mean to be of 0.9, thus:
[tex]M = 0.9[/tex]
[tex]\frac{4.2 + n}{5 + n} = 0.9[/tex]
[tex]4.2 + n = 0.9n + 4.5[/tex]
[tex]0.1n = 0.3[/tex]
[tex]n = \frac{0.3}{0.1}[/tex]
[tex]n = 3[/tex]
3 more testes are need, each worth 5 points, hence, 15 more points are needed to bring up his average up to 90%.
A similar problem is given at https://brainly.com/question/25323941