Answer:
The decay rate 'm' is 0.25
Explanation:
As given,
Average Life time of New cell phone = 4 years
Lifetime of phone = L = ?
L is a random variable defining lifetime of a cell phone.
L∼ Exponential (m)
Mean of Exponential Distribution =

As, they follow exponential distribution, so to find decay rate , we simply can write it as :
= 4
or

m=0.25
L∼ Exponential (0.25)
The decay rate is 0.25