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February 2nd, 2015, 05:44 AM   #2
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 Originally Posted by Smrithi Hi I wanted to share an interesting trick which i have read in a site. You can play this trick with your friends and prove your intelligence...... Ready???? Let's start Ask your friend to think of a number. Let him/her follow the below steps. 1. Add 2 with the number $\displaystyle \ \ \ \ \ \ \ \ \$ N 2. Multiply result with 5.$\displaystyle \ \ \ \ \ \ \ \ \ \ \ \$ 5N 3. Add 3 with the answer.$\displaystyle \ \ \ \ \ \ \ \ \ \$5N + 3 4. Multiply 2 to the resultant number and add it with four. $\displaystyle \ \ \ \ \ \ \ \ \$2(5N + 3) + 4 = $\displaystyle \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \$10N + 6 + 4 = $\displaystyle \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \$10N 5. Divide the answer with 10. $\displaystyle \ \ \ \ \ \ \ \ \ \dfrac{10N}{10} \ = \ N$ Now if your friend says the resultant value, you can guess the number thought by your friend!!!!!
.

 February 2nd, 2015, 05:52 AM #3 Global Moderator   Joined: Dec 2006 Posts: 20,926 Thanks: 2205 Your annotations feature some mistakes.
February 3rd, 2015, 08:22 AM   #4
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Quote:
 Originally Posted by Smrithi Ask your friend to think of a number. Let him/her follow the below steps. 1. Add 2 with the number 2. Multiply result with 5. 3. Add 3 with the answer. 4. Multiply 2 to the resultant number and add it with four. 5. Divide the answer with 10. Now if your friend says the resultant value, you can guess the number thought by your friend!!!!!
Let the guessed number = n

{2[5(n + 2) + 3] + 4} / 10

= (10n + 30) / 10

= 10(n + 3) / 10

= n + 3

So result always 3 more than the guessed number.

 February 3rd, 2015, 11:55 AM #5 Banned Camp   Joined: Jun 2014 From: Earth Posts: 945 Thanks: 191 Originally Posted by Smrithi View Post Hi I wanted to share an interesting trick which i have read in a site. You can play this trick with your friends and prove your intelligence...... Ready???? Let's start Ask your friend to think of a number. Let him/her follow the below steps. Let N = the guessed number. 1. Add 2 with the number N + 2 2. Multiply result with 5. 5(N + 2) = 5N + 10 3. Add 3 with the answer. 5N + 10 + 3 = 5N + 13 4. Multiply 2 to the resultant number and add it with four. 2(5N + 13) + 4 = 10N + 26 + 4 = 10N + 30 5. Divide the answer with 10. $\displaystyle \dfrac{10N + 30}{10} = N + 3$ N + 3 = three more than the guessed number. Now if your friend says the resultant value, you can guess the number thought by your friend!!!!! Last edited by Math Message Board tutor; February 3rd, 2015 at 12:00 PM.
 February 3rd, 2015, 01:34 PM #6 Math Team     Joined: May 2013 From: The Astral plane Posts: 2,257 Thanks: 928 Math Focus: Wibbly wobbly timey-wimey stuff. It's a nice little trick. It's even more fun in base 3. -Dan
 February 3rd, 2015, 02:48 PM #7 Global Moderator   Joined: Dec 2006 Posts: 20,926 Thanks: 2205 Alternatively, multiply by 10, then subtract 4, then divide by 2, etc. There is a better version of the trick that uses steps that cannot be "reversed" in that way to obtain the number used.
February 3rd, 2015, 05:12 PM   #8
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Quote:
 Originally Posted by skipjack Alternatively, multiply by 10, then subtract 4, then divide by 2, etc. There is a better version of the trick that uses steps that cannot be "reversed" in that way to obtain the number used.
What is that version?

 February 3rd, 2015, 07:35 PM #9 Global Moderator   Joined: Dec 2006 Posts: 20,926 Thanks: 2205 I forget its precise details, but it involved asking the user to introduce a second number of limited size, such as a year. The calculations had the effect of concatenating the two numbers. To make that unobvious, the number to be subtracted at the end was a 3-digit number.
 February 3rd, 2015, 08:14 PM #10 Math Team   Joined: Dec 2013 From: Colombia Posts: 7,674 Thanks: 2654 Math Focus: Mainly analysis and algebra I remember reading of one that involved getting three people to think of a number each. You take the first, do some operations, add in the second, do more operations, add in the third and do some more operations. Again, I believe there was a three-digit subtraction to be done to get the three numbers back again.

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