The graph shows a function that models the value V (in millions of dollars) of a stock portfolio as a function of time t (in months) over an 18-month period.

(a)
[tex]0\leq t\leq 4[/tex]
we can see that
curve is moving upward from t=0 to t=4
so, this is increasing interval.........Answer
(b)
[tex]15\leq t\leq 18[/tex]
we can see that
curve is moving upward from t=15 to t=18
so, this is increasing interval.........Answer
(c)
[tex]4\leq t\leq 15[/tex]
we can see that
curve is moving downward from t=4 to t=15
so, this is decreasing interval.........Answer