Let L be the number of hours worked on the lawn service and G be the number of hours worked on a gym.
Since he can work at most 20 hours and the total amount of hours worked is L+G, then:
Since Jonathan earns $10 for each hour working on lawn service and $8 for each hour working on the gym, the total amount of money that he earns is 10L+8G. Since he needs to earn at least $175, then:
These two inequalities make up the system that describes this situation.