[問題]PREP-T1-Q142 Q143 (PS)

由 **oneseven** » 2007-09-10 21:24

Q142
The rate of a certain chemical reaction is directly proportional to the square of the concentration of chemical A present and inversely proportional to the concentration of chemical B present. If the concentration of chemical B is increased by 100 percent, which of the following is closest to the percent change in the concentration of chemical A required to keep the reaction rate unchanged?

(A) 100% decrease
(B) 50% decrease
(C) 40% drcrease
(D) 40% increase
(E) 50% increase

Answer : D

Q143
In a survey of students, each student selected from a list of 12 songs the 2 songs that the student liked best. If each song was selected 4 times, how many students were surveyed?
(不是很懂題目的意思)
(A) 96
(B) 48
(C) 32
(D) 24
(E) 18

Answer : D

請教如何算這兩題呢謝謝

由 **eced** » 2007-09-11 01:37

Q142
R=k*A^2 /B (k為常數)
B'=2B
R'=R
what's A'?
k*A^2/B =k*A'^2/2B ; 2A^2=A'^2; A'=sqr(2)*A=1.4*A
so A should incluse 40%

Q143
每個學生從12首歌裡選出2首他最喜歡的,而每一首歌都被選了四次,問有多少學生

12*4/2=24 全部共有12*4首被選, 每個人只能選2首