Which math expression represents the phrase? 4 times the sum of a number and 9
a. 4 · n + 9
b. 4 · (n · 9)
c. 4 + (n + 9)
d. 4 · (n + 9)

Sandy is upgrading her internet service. fast internet charges $20 for installation and $52.45 per month. quick internet has free installation but charges $57.45 per month. part 1 out of 2 complete the equation that can be used to find the number of months after which the internet service would cost the same. use the variable x to represent the number of months of internet service purchased.