Alice and Bob
Today is Alice's birthday. She turned 24. Alice is twice as old as Bob was when Alice was as old as Bob now. How old is bob now ?
Labels: logic
posted by ToM at
11:16 PM
Thursday, June 11, 2009
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17 Comments:
First post!! the answer is 12
First Post doesn't always mean right answer.
Bob is 18.
how the heck did u get it i cant get any useful info frm the question.
the correct answer,as said above,is 18.
this is the way:-
let x be Bob's age now
let x-y be bobs age then.
hence Alice's age then=24-y which is also equal to x cuz Alice's age then=Bob's age now
and Alice's age now =2(x-y)=24
x-y=12
SO:
24-y=x
y=24-x
Substituting in other equation:
x-(24-x)=12
x=18
I went to bed with like 5 different answers and woke up with 18. I tried twice to prove it mathematically but failed. Good job SBA. Bob was 12(half of 24), When alice was 18(as old as bob is now). that is a six year difference which is maintained if bob is now 18 and alice is now 24.
Is alice pretty? My number is (if alice is reading) 7779998876.:P
here a riddle:
Whats greater than god, more evil than the devil and if u eat it u will die!EAT THAT!!!!!!!!!!losers
well it implys in your name that you are kooky/crazy so the only loser here is you, so shut ur your mouth (pointing to this guy who is retarded and thinks hes superman) and go back to where ever you came from, and if you were there in the first place, then just stop posting u big old kooky kook!!!
and that riddle is stupid, if it would kill you if u eat it, wat retard whould eat it, cause you said "eat this", oh im sorry you were talking to yourself wernt you, and thats the first sign of maddness so your mad/kooky/crazy and wat saddo would give there number to an imaginary person, man ur stupid
and i think the answer is 18 and koky didnt even give an answer
Tell it like it is, Matt. Why do these people bother to write a post with no intention of answering it.
Anyway, the answer to his stupid riddle is "NOTHING"
Nothing greater than God
Nothing more evil than the Devil
If you eat nothing you will die
Nothing is as annoying as Kocky
I got the answer 18 with help from my son
I think he is 36
the Alice age was 16 and Bob age was 8 now alice age is 24 and Bob age is 16
then the Alice age was the double of Bob age ......
kindman is right because if bob is 18 now, that would make him 6 years younger than alice, which makes sense for alice to be 12 when bob was six, BUT..... if alice was twice as old as bob was when she was how old he is now, than that would make him 9 at the time, causing him to be 17 now, which does not work. HOWEVER, 16 does because if bob is 16 now, that makes him 8 years younger than alice, so when she was 16 he would have to be half her age, 16-8=8, half, the age, 8 years younger, all requirements met!
Please try to read the question properly before answering. The answer is not 18 years old; its 16 years old as the people above me have stated... The question does not say 'Bob's age now' is half of 24 (which is 'Alice's age now'). It says that half of 'Alice's age before' (which is also 'Bob's age now') is 'Bob's age before'.
SO that means:
Let 'Bob's age now' = x
(This is what we are trying to find out)
Let 'Bob's age before' = x/2
(The question says Alice's age before was twice Bob's age before; in addition, Bob's age now is the same as Alice's age before. So in summary, Bob's age before was half his age now.)
'Alice's age now' = 24
'Alice's age before' = 24 - y
('y' is the number of years between 'before' and 'after')
Also
'Bob's age before'= x - y
(this can also be said for 'Bob's age before')
As stated above, there are two equations for 'Bob's age before', which are equal to each other. So:
x/2 = x - y
x/2 - x = -y
x/2 - 2x/2 = -y
-x/2 = -y
x/2 = y
And since 'Bob's age now' is equal to 'Alice's age before', it can be said:
24 - y = x
OK, since there is two variables ('y' and 'x') plus two equations, and we only need one variable to answer the question (which is 'x'), we needed to isolate the 'y' value in one of the equations so that we could replace the 'y' value in the other equation. And thus we would only have the 'x' variable left, which we could solve for after. Like this:
x/2 = y
24 - y = x
24 - (x/2) = x
24 = x + x/2
24 = 2x/2 + x/2
24 = 3x/2
48 = 3x
16 = x
This means Bob's age now is 16 years old. I have tried to make this whole math process as simple as I can. Its pretty long, so its probably better if you just use good old trial and error lol. There is nothing wrong with guessing, I myself used trial and error at first... but when I saw that the answer I got did not match the first few answers, I set out to prove, mathematically, that you guys are wrong (and consequently, the guys above this post were right =D).
Bob`s 18.
Bob's 12
@Hador: Sorry, but you're slightly off; the question says "Alice *is* twice as old as Bob was". You're solving "Alice *was* twice as old as Bob was".
Therefore:
A(now) = 24
B(then) = A(now)/2 = 12
A(then) = B(now)
A(now) - A(then) = B(now) - B(then)
24 - B(now) = B(now) - 12
36 = 2 * B(now)
18 = B(now)
If Alice is 24 and Bob is 18, then 6 years ago Alice was 18 and Bob was 12. Therefore Alice *is* twice as old as Bob *was* when Alice *was* as old as Bob *is*.
With the help of my TI-30,I got 18.
42
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