labour costs and remuneration methods

Employees are paid $7/hour for a standard 40 hour week. Overtime is paid at time and one half.
The employee is expected to make at least 120 units in a week, and to encourage productivity, each unit in excess of 120 will generate an additional payment of $5 less any overtime premium that would relate to the time the additional unit would normally take.
What is an employee’s wages in a week in which 135 units are made and the employee works 44 hours?

Example 1
Note: each unit is expected to take 40 x 60/120 = 20 minutes
$ Basic wages for 44 hours = 44 x $7 = 308.00
Overtime premium 4 x $7 x ½ = 14.00
Extra units 15 x $5 75.00
Overtime premium that could be associated with the extra units
15 x 1/3 x $7 x 1/2 -17.50 379.50

I still don’t understand where the 1/3 came from. What was the calculation made that made them included 1/3?