Question 975639
The equation for costs is 
{{{C(t)=3000+170t}}}
where t is measured in months.
The equation for revenues is,
{{{R(t)=420t}}}
Profit equals revenue minus costs,
Find when P reaches zero.
{{{P=R-C}}}
{{{P=420t-3000-170t}}}
{{{P=250t-3000}}}
{{{250t=3000}}}
{{{t=3000/250}}}
{{{t=12}}}{{{months}}}
So in the 13th month from starting she'll have a profit.