OK.
USUALLY when they give you this kind of graph (histogram),
each bar tells you the NUMBER of individuals that fall into
that particular range.
This graph is a little bit different ... it's easier.
On this graph, they're showing you the PROBABILITY that
an individual is in that particular range.
(The y-axis is labeled 'P', and they told us it's a "probability distribution".)
The graph is telling us that if you close your eyes and pick one giraffe,
he has a 25% chance of being 4 years old or younger, a 10% chance of
being between 4 and 8, a 35% chance of being between 8 and 12, and
a 5% chance of being between 16 and 20.
This is just a fancy, slightly confusing way of saying that 25% of the
giraffes in that zoo are 4 or younger, 10% of them are between 4 and
8, 35% of them are between 8 and 12, and 5% of them are between
16 and 20.
Before we worry about answering the question, we can spot a few
interesting things about the giraffe population at that zoo:
-- There are NONE that are between 12 and 16 years old. Weird.
-- The percentages only add up to 75%. So 25% of the giraffes
must be older than 20 ! They're just not included on the graph.
OK. So the question asks: What's the probability that a giraffe
you pick is between 8 and 20 ?
-- He's got a 35% chance of being between 8 and 12, no chance
at all of being between 12 and 16, and a 5% chance of being
between 16 and 20.
Can you see that his chance of being between 8 and 20 is
(35% + 0% + 5%) = 40% ?
