Problem :
Nathan and Joe are shopping for video games. Nathan's demand
function for video games is Q = 30 - 3P, and Joe's demand function
is Q = 48 - 4P. What will their combined demand be if the price is
$5? $11?

If we add Nathan and Joe's demand functions, we get:
At $5 a game, both Nathan and Joe will have positive demand for
video games, and so we can use the combined equation to get

Q = [78 - 7(5)] = 43 games.

At $11 a game, however, Nathan's demand function gives negative
demand, which we know means he just has 0 demand for video games.
In this case, we ignore Nathan's function, and just use Joe's to
figure out their combined demand, since using the combined function
would give the wrong answer.

Q = [48 - 4(11)] = 4 games.

Problem :
Michelle is shopping for shirts. She chooses one, then notices that
the shirts are on sale, and gets another two shirts. How can you
explain this with a graph?

This is an example of moving along a demand curve. Nothing except
for the price has changed, so when Michelle notices the price
change, she buys more shirts.

Problem :
Jenn's parents increase her allowance, so she spends more money on
candy every week. How can you explain this with a graph?

This is an example of a shift in Jenn's demand curve. She has a
permanent increase in her income, so she begins to buy more, even
though the prices haven't dropped.

Problem :
Kris and Tim's demand curves for playing cards look like this:

Who wants more when the price is $3 a pack? $4 a pack? What is
their combined demand at $4 a pack?
As we can see on the graph, Kris will buy more than Tim if the price is $3 a
pack, but if the price is $4 a pack, they will have equal demand for playing
cards. At $4 a pack, they will each buy 3 packs, for a combined demand of 6
packs.

Problem :
Emily decides to set aside $200 from her paycheck every month. How
will this affect her demand curve? (You don't have to use specific
numbers, just explain)

When Emily sets aside money from her paycheck, this is the same
thing as a
decrease in her income. Remember that when income increases, the
demand curve shifts outwards to reflect the increase in spending. In
this case, Emily's demand curve will shift inwards as she tries to
economize more than usual. She will buy fewer goods, even if the
prices don't change.