Question 193959
Your answer to the first part is correct!
For the second part, you need to set the total cost, c = $90 then solve for t, thus...
{{{c = 30t+20}}} Substitute c = 90
{{{90 = 30t + 20}}} Subtract 20 from both sides.
{{{70 = 30t}}} Now divide both sides by 30.
{{{70/30 = t}}} so...
{{{t = 2.333}}}
For $90 the customer can get 2 1/3 months of service.
Check:
{{{c = 30t+20}}} Substitute t = 2 1/3
{{{c = 30(2.333)+20}}}
{{{c = 70+20}}}
{{{c = 90}}}