Answer
$5.616 billion
Work Step by Step
We assume that $60\%$ of the $7.8$ million Mexican-born immigrants remit money to family members, which constitutes
$0.60\times 7.8=4.68$ million people,
and if each person sends ${$100}$/month, or ${$1200}$/year, then the total amount of money remitted is
$1200\times 4.68=\$5.616$ billion