Answer:
a) There is evidence that more than 20% of the customers would upgrade to a new cellphone at a reduced cost
b) The manager in charge of promotional programs concerning residential customers will help the customers to upgrade their cellphones at a reduced cost.
Explanation:
sample size, n = 500
The number of subscribers who would upgrade to a new cellphone at reduced cost, X = 135
Proportion of those that would upgrade to a new cellphone,

The null hypothesis is that less than or equal to 20%( i.e. 0.2) of the customers would upgrade to a new cellphone at a reduced cost while the alternative hypothesis is that more than 20% would upgrade to a new cellphone at a reduced cost.
Null hypothesis,

Alternative hypothesis,

The test statistic can be calculated as thus:



To get the p - value for the test statistic:

Since the p - value is less than 0.05 significance level, the null hypothesis will be rejected. (i.e. more than 20% would upgrade to a new cellphone at a reduced cost.)