Today, Darren has ECON 301 exam in the morning and sociology exam in the evening. Yesterday night, Darren had time to ready 40 pages of economics and 30 pages of sociology. Alternatively, he had time to read 10 pages of economics and 90 pages of sociology. Which of these equation describes all combination of pages of economics, E, and sociology, S, that he could read?

a) E/2 + S = 50

b) E + S = 100

c) E + S =70

d) 2E + S = 110

e) All of the above