Answer:
I think it's the 2nd choice. Hopefully I got this correct :)
Explanation:
I think you wrote that z will represent the number of hours spent babysitting but I think you meant x.
It's the 2nd choice because $30 is the flat rate for Julia and $10 is the flat rate for Kelsey. Julia has a rate of $5 per hour so that will be the number with the x. And Kelsey has a rate of $15 per hour so that will be the number with the x.
Julia: y = 5x + 30
Kelsey: y = 15x + 10.