请高手 帮忙解释一下第三章3（10）C  图的原因：题目如下

3.10 (2) Ralph Rigid likes to eat lunch at 12 noon. However, he also
likes to save money so he can buy other consumption goods by attending
the \early bird specials" and \late lunchers" promoted by his local diner.
Ralph has 15 dollars a day to spend on lunch and other stu . Lunch at
noon costs $5. If he delays his lunch until t hours after noon, he is able
to buy his lunch for a price of $5−t. Similarly if he eats his lunch t hours
before noon, he can buy it for a price of $5−t. (This is true for fractions
of hours as well as integer numbers of hours.)
(a) If Ralph eats lunch at noon, how much money does he have per day
to spend on other stu ? $10.
28 PREFERENCES (Ch. 3)
(b) How much money per day would he have left for other stu if he ate
at 2 P.M.? $12.
(c) On the graph below, use blue ink to draw the broken line that shows
combinations of meal time and money for other stu that Ralph can just
a ord. On this same graph, draw some indi erence curves that would be
consistent with Ralph choosing to eat his lunch at 11 A.M.

图在附件中