求助！！！！看看大家是怎么理解这句话的!！！！！

lovesiqi4665

the average age of the United States population will increase between 1987 and 2000.
My understanding to this setence is that the average age  is increasing; that is, Americans is becoming older. and i think the prep "of“ served as a restrictive to "the average age"  
But, the official guidelines give the alternative meaning of the sentence aboved that the people who reach average age are increasing.
Thus，who can help me in explaining the disparity between the meaning of mine and officials' ? Thanks！

vansonkated

i agree w/ u. the subject is age, not population, so if make the sentence short, the age will increase, so... i guess the book is wrong? is it really that matter???

lovesiqi4665

Trust me, i wouldn't get the meaning of the official's wrong, maybe we are both wrong...hehe
If possible, you can search for the OG10th CR 174th question, and examine the explaination of choice C the official guideline provided. if you can do that, thanks a lot !!!

queenie924

In 1987 sinusitis was the most common chronic medical condition in the United States,
followed by arthritis and high blood pressure, in that order.
The incidence rates for both arthritis and high blood pressure increase with age, but the
incidence rate for sinusitis is the same for people of all ages.
The average age of the United States population will increase between 1987 and 2000.
Which of the following conclusions can be most properly drawn about chronic medical
conditions in the United States from the information given above?

A. Sinusitis will be more common than either arthritis or high blood pressure in 2000.

B. Arthritis will be the most common chronic medical condition in 2000.

C. The average age of people suffering from sinusitis will increase between 1987 and2000.

D. Fewer people will suffer from sinusitis in 2000 than suffered from it in 1987{#}

E. A majority of the population will suffer from at least one of the medical conditionsmentioned above by the year 2000.

Note: same disease rate does not mean there will be the same quantity of sick people since the total number of the population might change.  Same disease rate simply means that the ratio between the sick among each age group is constant.

Having said that, this question is purely a math problem. Since the rate for sinusitis is independent of age, then the average age of patient having sinusitis is simply the average age of the whole population.

Suppose there are p people with the age of a, and within this age group there are s people who has sinusitis.  The age span is from a1 to an.

rate (of sinusitis) = s1/p1 = s2/p2 = s3/p3 = . . . = sn/pn

average age for the population = (a1*p1 + a2*p2 + a3*p3 + . . . + an*pn) / (p1 + p2 + p3 + . . . + pn)

average age for patients = (a1*s1 + a2*s2 + a3*s3 + . . . + an*sn) / (s1 + s2 + s3 + . . . + sn)
= (a1*p1*rate + a2*p2*rate + a3*p3*rate + . . . + an*pn*rate) / (p1*rate + p2*rate + p3*rate + . . . + pn*rate)
= [rate/rate][(a1*p1 + a2*p2 + a3*p3 + . . . + an*pn) / (p1 + p2 + p3 + . . . + pn)]
= (a1*p1 + a2*p2 + a3*p3 + . . . + an*pn) / (p1 + p2 + p3 + . . . + pn)
= average age for the population

lovesiqi4665

In 1987 sinusitis was the most common chronic medical condition in the United States,
followed by a ...
queenie924 发表于 2011-7-12 21:42

    I got it !
You are the best！！！thank you very much for your detailed explanation!!