Your friend chooses an integer between 0 and 100. You have to find out that number by asking questions, but your friend can only answer ‘Yes’ or ‘No’.

Question

Guess my number : 

Your friend chooses an integer between 0 and 100. You have to find out that number by asking questions, but your friend can only answer ‘Yes’ or ‘No’. What strategy would you use ?

Answer 1

Let my friend choosen 73; then my questions may be like this.

Q : Does it lie in the first 50 numbers ? 

A: No . 

Q : Does it lie between 50 and 60 ? 

A: No 

Q : Does it lie between 60 and 70 ? 

A: No 

Q : Does it lie between 70 and 80 ? 

A: Yes 

[then my guess would be “it is a number from 70 to 80] 

Q : Is it an even number ? 

A: No 

[my guess : it should be one of the numbers 71, 73, 75, 77 and 79]

Q : Is it a prime number ? 

A: Yes 

[my guess : it may be 71, 73 or 79] 

Q : Is it less than 75 ? 

A: Yes 

[my guess : it may be either 71 or 73] 

Q : Is it less than 72 ? 

A: No 

Then the number is 73. 

Therefore the strategy is → Use number properties such as even, odd, composite or prime to determine the number.