A storage company has two types of units: small and large. Each small unit costs $100 per month, while each large unit costs $150 per month.
The company has a total of 30 units rented out, earning a total of $4,200 in monthly revenue from these units. Let s represent the number of small units and l represent the number of large units.
Which of the following equations can be used to find the values of s and l?