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gmatclub electronic clock

by arora007 » Tue Jul 06, 2010 11:47 am
An electronic clock shows time in hours and minutes. For example, 08:15 means that it is now a quarter past eight. If one looks at the clock at some time between 6:00 and 9:00, what is the probability that he will see the digit 8 ?

a) 1/5
b) 1/3
c) 2/5
d) 4/9
e) 1/2

C
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by kvcpk » Tue Jul 06, 2010 11:55 am
An electronic clock shows time in hours and minutes. For example, 08:15 means that it is now a quarter past eight. If one looks at the clock at some time between 6:00 and 9:00, what is the probability that he will see the digit 8 ?

first time it shows is 6:08
then 6:18,6:28...38,48,58
6 cases out of 60 between 6 and 7
6 cases out of 60 between 7 and 8
now between 8:00 and 9:00 he will see all the time. All the 60 cases.

So 72 cases out of 180 cases

=72/180 = 2/5

Hope this helps
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by adi_800 » Fri Jul 09, 2010 8:06 am
I think the problem is not worded properly...
Because the observer just wants to see 8, then 8:17 is a valid case and 8:18 is also a valid case.

In the case of 8:18, he sees the 8 twice and not once...
So, if we take that into consideration, there are more possibilities..
But the answer that we get considering these is not present in the option...
So, its fine if we dont consider them...
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