If Danny has AT LEAST $15 dollars more than his brother, and his brother has $72, then the equality would start out like this (where d = the amount of money Danny has):
[tex]d\ge72+15[/tex]
Because he has AT LEAST $15 more; it could be exactly 15, or it could be any number more than 15. So, to simplify:
[tex]d\ge72+15\\d\ge87[/tex]
Danny has at least $87 ($87 or more).