Hi,
Let x, (x + $112) and (x+ $306) represent Hunter, Mackensie and Grace amount.
Question states
x + (x + $112) + (x+ $306) = $1312
solving for x
3x + $418 = $1312
3x = $894
x = $298, Hunter's amount. Grace paid $604
CHECKING our Anwer
$298 + $410 + $604 = $1312