$250 - in his account
$35 per month out of (minus $35 per month from his account)
$250 - 1 · $35 - after the first month
$250 - 2 · $35 - after the second month
$250 - 3 · $35 - after the third month
...
n - the number of a month
The function:
<h3>f(n) = 250 - 35n</h3>
Domain:
250 - 35n > 0 <em>subtract 250 from both sides</em>
-35n > -250 <em>change the signs</em>
35n < 250 <em>divide both sides by 35</em>
n < 7.142.... → n < 8 → n ∈ {0, 1, 2, 3, 4, 5, 6, 7}