The sum of all the measurements is just the average times the number.
[tex]\bar x = \frac{1}{n} \sum_{i=1}^n x_i [/tex]
[tex] \sum_{i=1}^n x_i = n \bar x [/tex]
So if the average of all 92 is 7 the sum of those is [tex]92 \times 7[/tex]
The average of the last two is 7.2 so their sum is [tex]2 \times 7.2[/tex]
That means the sum of the first 90 is [tex]92 \times 7 - 2 \times 7.2[/tex]
so the average of the first 90 is
[tex]\dfrac{92 \times 7 - 2 \times 7.2}{90} = 6.99556[/tex] cm
