请教PREP数学题

emilyxiang

152.
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A certain jar contains only b black marbles, w white marbles, and r red marbles.  If one marble is to be chosen at random from the jar, is the probability that the marble chosen will be red greater than the probability that the marble chosen will be white?

 

(1)r/b+w >w/b+r
                        

 

(2)  b – w > r

答案是A

 

140.
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A manufacturer produced x percent more video cameras in 1994 than in 1993 and y percent more video cameras in 1995 than in 1994.  If the manufacturer produced 1,000 video cameras in 1993, how many video cameras did the manufacturer produce in 1995 ?

 

(1) xy =
                20

 

(2) x + y + xy/100 = 9.2

答案是B

 

麻烦各位告诉我该怎么做？不甚感激！！

 

 

tigerflying

152. The probability of red: P(r)=r/(b+w+r), and the white P(w)=w/(b+w+r), so if (r)>(w), then r>w.

(1).r/b+w>w/b+r,-->(r-w)(1-1/b)<0 , because b is positive integer, so (r-w)<0.

(2) b-w>r  --->b>w+r, could not get the result r>w.

so the answer should be A

140.From the information, we could get V94=V93(1+x%), and V95=V94(1+y%), so V95=V93(1+x%)(1+y%)=V93(1+x+y+xy%)%, so (2) i enough.  

emilyxiang

thank