How old is Ann? • Hacker News Board

Re: How old is Ann?

#11

Post by lukewarm707 » Thu, Jul 30, 2026, 1:58 PM UTC

Cockbrand wrote: post #7

nly wrote: post #2

Opus says Ann is 18

Sounds about right. How I understand it: Mary is 24, Ann is a few (n) years younger M = 24 A = M-n Mary is twice as old as Ann was when Mary was as old as Ann is now. So we have to deduct the age difference twice from Mary's current age to find out how old Ann was when Mary was as old as Ann is now, which is half Mary's current age: M-2n = M/2 = 12 M-2n = 12 --> n=6 So the age difference is 6, and Ann is 18.

mine was not so parsimonious, my teachers would probably say i am not very clever but has points for effort.

ann_old=12

mary_curr=24

mary_old=ann_curr

mary_curr-ann_curr=X

mary_old-ann_old=X

24-ann_curr=X

mary_old-12=X

ann_curr-12=X

24-ann_curr=ann_curr-12

2ann_curr -12 = 24

2ann_curr = 36

ann_curr = 18

Re: How old is Ann?

#12

Post by [deleted] » Thu, Jul 30, 2026, 2:00 PM UTC

tines wrote: post #5

Wait so how old is she?

[deleted]

Re: How old is Ann?

#13

Post by tempfile » Thu, Jul 30, 2026, 2:01 PM UTC

Mary IS 24. So Ann WAS 12. Say that was X years ago. Then Mary WAS 24-X and Ann IS 12+X. So 24-X=12+X, X=6, Ann is 18.

I can't see a way to do it without algebra.

Re: How old is Ann?

#14

Post by swiftcoder » Thu, Jul 30, 2026, 2:03 PM UTC

lordgrenville wrote: post #8

"Any eighth-grade boy or girl who is 'all there' should solve it in about thirty seconds" is hyperbole (it took me a couple of minutes), but in general the public reaction to this seems like a demonstration of the Flynn effect.

> it took me a couple of minutes

Are you in the 8th grade? Those of us a couple of decades past the 8th grade are maybe slower at algebra than someone who is actively drilling it...

Re: How old is Ann?

#15

Post by aetherson » Thu, Jul 30, 2026, 2:10 PM UTC

feelamee wrote: post #6

what the true answer? oh, there is no? My intuition says that Ann now is 24

18.

Re: How old is Ann?

#16

Post by NooneAtAll3 » Thu, Jul 30, 2026, 2:14 PM UTC

tempfile wrote: post #13

Mary IS 24. So Ann WAS 12. Say that was X years ago. Then Mary WAS 24-X and Ann IS 12+X. So 24-X=12+X, X=6, Ann is 18. I can't see a way to do it without algebra.

there's 12 years between 12 and 24. it is divided equally between OldAnn-OldMary and OldMary-NowMary. 12/2=6, 12+6=18

Re: How old is Ann?

#17

Post by zephen » Thu, Jul 30, 2026, 2:19 PM UTC

tempfile wrote: post #13

Mary IS 24. So Ann WAS 12. Say that was X years ago. Then Mary WAS 24-X and Ann IS 12+X. So 24-X=12+X, X=6, Ann is 18. I can't see a way to do it without algebra.

> I can't see a way to do it without algebra.

This is related to "When all you have is a hammer, everything looks like a nail."

It's slightly different, because you've supercharged your hammer. Everything you ever understood about how to solve word problems has been subsumed into what your brain labels as "algebra."

But look at it this way:

1) Did you use algebra to convert the problem to algebraic form? Probably not; algebra says nothing about word problems.

2) Once it was in algebraic form, did you need to repeatedly apply algebraic rules in order to reduce the problem, or could you glance at it and figure it out?

You may also be hampered by your choice of variable, because you chose "X" to be an intermediate variable.

If, instead, you choose "X" to be what you are searching for, Ann's current age, then the problem setup is:

  24 - x = x - 12

Which many of us can solve in our heads without writing down, or even without consciously converting "Ann's current age" to "X".

Re: How old is Ann?

#18

Post by zephen » Thu, Jul 30, 2026, 2:20 PM UTC

nly wrote: post #2

Opus says Ann is 18

But that was over a hundred years ago. She must be dead by now.

Re: How old is Ann?

#19

Post by Hackbraten » Thu, Jul 30, 2026, 2:23 PM UTC

> When Mary had a little lamb, she was twice as old as Ann was when Ann was as old as Mary now claims to be. How old is the sheep? —James Barton Adams.

I love this response so much.

Re: How old is Ann?

#20

Post by n6242 » Thu, Jul 30, 2026, 2:23 PM UTC

swiftcoder wrote: post #14

lordgrenville wrote: post #8

"Any eighth-grade boy or girl who is 'all there' should solve it in about thirty seconds" is hyperbole (it took me a couple of minutes), but in general the public reaction to this seems like a demonstration of the Flynn effect.

> it took me a couple of minutes Are you in the 8th grade? Those of us a couple of decades past the 8th grade are maybe slower at algebra than someone who is actively drilling it...

I must be getting old as well because I had to write it down in a piece of paper to do it, but I did get it pretty quick.