 Thursday, May 6, 2010

Genius Grandkid

Here is a trick that could help you and your little kid impress a grandma this Mother's Day. A first or second grader is good as they know subtraction, but being able to also do large division wouldn't be suspected. Allow your grandma to use a calculator. Ask her to pick any number. Multiply it by 2, add 5, multiply by 5, and announce the result.

Teach your kid to write the resulting number on a piece of paper. Cross the last digit from this number and subtract 2 from the remaining number to get the number that grandmother picked.

For example: she picks 21.
21 x 2 + 5 = 47
47 x 5 = 235

Your kids takes 235. Crosses out the 5, obtaining 23.
23-2=21

He is genius!
Now, you could be genius if you figure out how does this trick work?

(this trick is from B. Kordemsky book)

Submit your answer on our Family Puzzle Marathon Be first to solve three puzzles and get a prize!

rahul said...

Say the number picked by grandma is x. When you double it and add 5 you get 2x + 5. When you double that you get 10x + 25. Crossing the last digit from this number is equivalent to dividing by 10 and crossing out the numbers after the decimal point. This gives you x + 2. Subtracting 2 from this number gives you x.

rahul said...

Edit to my answer given above: Where I've said "When you double that you get 10x + 25", I should have said "When you multiply that by 5 you get 10x + 25".

Note to self: Preview before posting.

Maria said...

Hey Rahul - you are a genius! Welcome to the puzzle marathon and congratulations on your first puzzle point.

Lynnet said...

You take the number n
multiply the number by 2 2n
multiply by 5 5(2n+5)= 10n+25
divide by 10 and drop fractions 10n/10 25/10= n+2.5 =n+2
subtract 2 n+2-2=n

the answer is n, or the number you started with.

Maria said...

That is correct. Bravo, Lynnet!
As Rahul posted his answer first, I can;t give you a puzzle point. But I see that you just proposed a faster solution for The Water Puzzle!