In all of these basically remember the order of precedence
1) Parenthesis
2) Exponent
3) Mult/Div
4) Add/Sub
5) Comparisons
ok, so the first expression:
4 + $c / 4 * $d
4 + 25 / 4 * 3
4 + 75 / 4
4 + 18.75
22.75 <-- first line
$a / $d * $a + $c
15 / 3 * 15 + 25
5 * 15 + 25
75 + 25
100 <--...