请教小蓝上三道数学题

zhangjy · 发表于 2010-12-16 16:31:32

1.bacteria quadruples everyhour. If a container with these bacteria was half full at 10:00 am. at what time was it one eighth full?

answer:9:00am..

2.if y is not equal to 3, and 3x/y is a prime integer greater than 2, which of the following must be true?
I.x=Y
II Y=1
IIIx and y are prime integers
A none
b I only
c II only
D III only
E i and III
answer: A, my question is why I is not correct, when X=y, 3x/y= 3 is is a primer integer than 2...

3.a coin that is tossed will land heads or tails, and each outcome has equal probability. what is the probability that the coin will land heads at least once on two tosses?
answer:1/4, why not 3/4 since 1-1/2*1/2=3/4,

thank you thank you thank you!

fanfantastic · 发表于 2010-12-16 16:47:22

细菌以每小时4倍的速度增长，早上十点的时候细菌占容器体积的1/2，问细菌在什么时候占容器提及的1/8.
所以是早上九点……从10点往前一个小时，1/2/4=1/8

zhangjy · 发表于 2010-12-17 20:06:01

up!!!!!!

chris625 · 发表于 2011-3-7 20:37:07

第三题吧，应该看做是一枚硬币扔两次，看有多少种正反组合；第一次正，第二次反；第一次反，第二次正；第一次第二次都正；第一次第二次都反；所以是3/4的概率~~太绕了.............

请教小蓝上三道数学题

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