Hi everyone,
 
Me and my friends came up with a fun trick today and I want to share it with you guys , I dunno if it's an old or well known trick , I want to share it nevertheless.It goes like this:
 
Ask someone to think of a family of X boys and Y girls , either or both of them can be zero.Now , ask the person to perform these steps:
 
1. add 2 to the number of  boys [X]
2. whatever result comes , multiply it with 2
3. now add 1 to the result
4. multiply the result with 5
5. finally ,add number of girls to it [Y]
 
now ask the person to tell you the total sum , the total sum is ,for instance, m.
now ,the trick is that you subtract 25 from m and you'll get a number of either one or two digits.
suppose you get , 45 then X=4 and Y=5, tell the person and he'll be surprised.In case ,the number you get is less than 10 (say 6) then  
6=06 , i-e X=0 and Y=6.
 
 
Trick Uncovered
 
If you know basic algebra , then the trick is quite fun to discover.You ask the person to perform these tasks with X:
 
5[2(X+2) + 1] + Y = m
solve this equation and you'll get:
10X + Y + 25 = m -------> (i)
 => 10X + Y = m - 25
(m - 25) is always greater than or equal 0 , i-e m is always greater than or equal to 25.
putting m - 25 = n , then above equation becomes:
10X +Y = n -------> (ii)
 
Since X and Y are family members , then it's natural for people to suggest values between 0-10 for both of them.
 
In eq(ii) , if you put any value of X (probably less than 10) then for every Y<10 , the first digit of n will clearly represent X and the second digit will represent Y.
 
The trick fails if Y>or=10 , that's why it's wise to select a family for this trick.
 
I hope I was clear enough to make you understand it .

Now I don't mean to sound too proud and pompous, but I actually discovered the trick by reading the question. Mostly because I practised plenty of such questions for my SAT Math Level 2. They don't really ask these, but it's a good way to boost your logic and algebra skills.
 
I did not, however, realize the Y>=10 catch till you mentioned it, though it now seems obvious.
 
I have a few of my own, I'll post them later.

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Glomping the internet populace.

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glad to know you got it correctly