50 employees work 40 hours/week at $20/hour.
Their combined regular hours salary is 50 * 40 * $20 = $40,000.
15 employees worked 4 hours at $30/hour.
Their combined overtime salary is 15 * 4 * $30 = $1,800.
The total payroll is the regular hours plus the overtime:
$40,000 + $1,800 = $41,800
The overtime is the following percentage of the total payroll:
$1,800/$41,800 * 100% = 4.3%
Answer: 4.3%
