Part C:
y = total cost
M = minutes
You can talk on the phone only 100 minutes per month
Company A:
y = 0.04M + 5
y = 0.04(100) + 5
y = 4 + 5
y = 9 $9 per month
Company B:
y = 0.10M + 2
y = 0.10(100) + 2
y = 10 + 2
y = 12 $12 per month
Company A offers the best deal because at Company A you have to pay $9 for 100 minutes per month, and at Company B you have to pay $12 for 100 minutes per month, so you have to pay $3 less.
Part D:
1.) With a budget of $30, Company A would allow me to talk longer on the phone. I know this because for Company A, you pay $3 less per month for the same amount of minutes as Company B. This means that I will save more money with Company A, and I can buy more minutes. (something like this)