Answer:
1) at 24.64 months 25% of the cellphones battery will fail
2) there is a 22.66% chance it will use the warranty
3) the expected return is 16.01 dollar of gain per warranty
Explanation:
at 25% we have to look into the normal distirbution table for the Z value that acumulates that
we find out is -0.67
now we solve for X value:
[tex]\frac{X - 30}{8} = -0.67\\\frac{X - \mu}{\sigma} = P_z[/tex]
X = 24.64
we solve for the provability when X = 24
we convert into a z value and look for the value inthe table
(24 - 30) / 8 = -0.75
probability = 0.226627352 = 22.66%
expected return
[tex]\left[\begin{array}{cccc}State&Return&Probability&Weight\\not use&50&0.7734&38.67\\use&-100&0.2266&-22.66\\&&&\\Total&&1&16.01\\\end{array}\right][/tex]