.
Let C(t) be the non-linear function defined by these two lines
C(t) = 500, if 0 <= t <= 2,
C(t) = 500+100*(t-2), if t > 2.
where t is the time in hours.
Then the inequality the problem asks for is
C(t) <= 900.
The solution set for this inequality is the time interval [
,
], which is [0, 6].