Hi Ritta,
To break it all down for you in simple terms:
$10.50 x 40 hours worked (initially) = $420 gross pay
$498.75 - $420 = $78.75 remaining
1.5 x $10.50 = $15.75/hour earned for every hour of overtime worked.
$78.75 / $15.75 = 5 hours of overtime worked.