So,
The rate David makes tea is 30 c./hr.
The rate Andrew makes coffee is 35 c./hr.
I am still waiting for a response to my question, so I'll set-up an equation.
Let the number of hours David works per week be D.
David makes 30 c. of tea per hour.
Multiply the number of hours David works in a week by D, and you will have the amount of tea he made in the week.
Add 10 to the number(a), and you will find the total amount of tea David made during the week.
30(D) + 10 = a
Do the same thing for Andrew, but this time don't add 10 to the penultimate number.
35(A) = b
Combined, you get
(30[D] + 10) + (35[A]) = c
