Answer
$a_n=150-0.75(n-1)$
Work Step by Step
We have the following table of the target heart rate for aerobic exercise at several ages.
Age $\quad$ Target Heart Rate
$20$ $\quad\quad$ $150$
$40$ $\quad\quad$ $135$
$60$ $\quad\quad$ $120$
$80$ $\quad\quad$ $105$
Assuming the values in the table form an arithmetic sequence where $n=1$ corresponds to age $20$ and $n=2$ to age $21$ we have to find the general term.
If $n=1$ corresponds to age $20$ then age $40$ corresponds to $n=21$. So we have arithmetic sequence where
$a_1=150$ and
$a_{21}=135$.
We have to find $d$.
$a_{21}=a_1+d(21-1)$ or $135=150+d\cdot20$
$d=-\dfrac{15}{20}=-0.75$
So the general term is $a_n=150-0.75(n-1)$
