Exam average

The average of marks that have on the certificates students of 8A class in mathematics is exactly 2.45. If we did not add 1 and 3 of students Michael and Alena, who arrived a month ago, it would average exactly 2.5.

Determine how many students have class 8A.

Correct answer:

n =  20

Step-by-step explanation:

$$\begin{gathered} 2.45=\frac{S}{n} \\ 2.5=\frac{S-1-3}{n-2} \end{gathered}$$

n · 2.45 = S
(n-2) · 2.5 = S-1-3

S-2.45n = 0
S-2.5n = -1

Row 2 - Row 1 → Row 2
S-2.45n = 0
-0.05n = -1


n = -1/-0.05 = 20
S = 0+2.45n = 0+2.45 · 20 = 49

S = 49
n = 20

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