<u>Answer-</u>

<u>Solution-</u>
Here, n represents the number of months and f(n) represents the number of laptops in the store after n months.
As the store had 150 laptops in the month of January or at the beginning.
So 
Every month, 20% of the laptops were sold and 10 new laptops were stocked in the store.
As 20% of laptops were sold, so 80% were in the store.
So, after one month total number of laptops in the store,


Again after one month total number of laptops in the store,


Analyzing the pattern, the recursive function f(n) will be,

To solve this problem, we make use of the z statistic. The formula for the z score is:z score = (x – u) / swhere x is the sample value = 0.90, u is the sample mean = 0.917, and s is the standard deviation = 0.005
Therefore:z score = (0.90 – 0.917) / 0.005z score = -3.4
From the standard probability tables, the p-value for a right tailed test of z = -3.4 is:P = 0.9997
Therefore there is a 99.97% chance that it will be above 0.90 mm
B= 30
a2+b2=c2
72^2+b^2 = 78^2
5184+b^2 = 6084
b2 = 900
b = 30
See Quadratic Formula and Determinant's/Delta's formula