Explanation:
if we put the amount of hours and how much Justin has been paid into a ratio we will get this:
$318.75 : 25 hours
in order to find out how much he has been paid for just one hour of work we must divide both sides by 25.
this is because 25 hours / 25 = 1 hour. let's try that:
$318.75 / 25 = $12.75
and 25 hours / 25 = 1 hour
let's put these values back into our ratio:
$12.75 : 1 hour
as you can see we have worked out the total amount Justin made in 1 hour of work, aka how much he earned in an hour.
if you need any more help, just ask :D