Annalea:

(a) wants a graph of how she could spend the 5 hours studying. one axis would have the pages of econ, the other would have sociology. on one end of the graph, she would spend the whole 5 hours reading econ(100 pages), and 0 hours reading sociology(0 pages). on the other end of the graph, she'd spend 0 hours reading econ (0 pages), and 5 reading sociology (250 pages). you'd draw a line from one point to the other.

here is an image of the problem:

http://cdis.missouri.edu/exec/data/c.../UFig02_03.gif
(b) is asking : how long does it take for Maria to read 100 pages of sociology? if she can read 50 pages in one hour, then that means it would take her 2 hours to read 100 pages.

now, in that same 2 hours , how many pages of economics could she have read? well, according to the problem, at 20 pages per hour, she could have read 40 pages of economics in that 2 hours.

so the opportunity cost is 40 pages of economics.