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zainab
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probability 08 Dec 2005, 06:14
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probabilty is giving me the creeps.somebody help me with this question.)A box contains 5 blocks numbered 1,2,3,4 & 5.Johnnie picks a block & replaces it.Lisa then picks up a block.What is the probability that the sum of the numbers they picked is even?
a)9/25
b)2/5
c)1/2
d)13/25
e)3/5
the answer is D :(



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dogbert
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probability 08 Dec 2005, 07:11
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The answer is 13/25....
Explanation:
For the total to be even, for every odd numbered block one picks, the second one has to pick another odd numbered block..
Thus the combinations can be:
1+1; 1+3; 1+5; 3+1; 3+3; 3+5; 5+1; 5+3; 5+5 = 9
For even numbers as well, a similar procedure:
2+2; 2+4; 4+2; 4+4 = 4

Thus: 9+4=13
And there are 5x5 ways to pick blocks...
Thus probability = 13/25..

I realise that the process above may be time consuming.. is there any shorter method to do it?
You could write:
Prob = (3x3 + 2x2)/5x5 =13/25
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HIMALAYA
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probability 08 Dec 2005, 07:33
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total = 5x5 = 25
sum is even = 13 (1+1, 1+3, 1+5, 2+2, 2+4, ..............., 5+5) however you can do alternatively too.
prob = 13/25.
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GMATT73
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probability 08 Dec 2005, 09:02
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dogbert

You could write:
Prob = (3x3 + 2x2)/5x5 =13/25
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This has got to be the fastest way.
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strange
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probability 08 Dec 2005, 09:14
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GMATT73
dogbert

You could write:
Prob = (3x3 + 2x2)/5x5 =13/25

This has got to be the fastest way.
Show more


This is how I did, if anyone can tell me if it's a good method..

The only way, an even sum can be obtained is if numbers picked up by Johnie & Lisa is either even or odd.

If Johnie picks up an even number, then Lisa should also pick an even number. Likewise, if Johnie picks up an odd number, Lisa should pick an odd number so that the sum of the numbers is even.

Prob of picking up an even number - 2/5 (2 & 4 are even in the list)
Prob of picking up an odd number - 3/5

If Johnie picks up even, then Lisa needs to pick even (as Johnie replaces the number he picks)
2/5 * 2/5 = 4/25
If Johnie picks up odd, then Lisa needs to pick odd
3/5 * 3/5 = 9/25

Summing them.. 4/25 + 9/25 = 13/25



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