26-09-2005, 11:39 AM
Quote:
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Originally Posted by sapanjain
Plz post the complete solutions to following problems :
3.There are 6561 balls out of them 1 is heavy.Find the min. no. of
times the balls have to be weighed for finding out the haevy ball.
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well, the
problem 2 doesnt specify whether to use a spring balance or a 2-pan balance, since answer would differ for both of them.
considering 2-pan balance, answer would be 8 times.
solution:
assume the no. of balls to be 3. it is obvious that u ll use the balance once. weigh two balls. if they weigh different, u get the heavier one. if they weigh same, 3rd ball is heavy.
take different cases of 4,5,...,8,9 balls. u will find in all the cases, u ll have to weigh two times, making groups of balls and weighing them together. this can be generalised like:
no. of balls -------------
no. of weighing
1 to 3 i.e. 3^1 -----------------1
4 to 9 i.e. 3^2----------------- 2
10 to 27 i.e. 3^3 --------------3
28 to 81 i.e. 3^4 --------------4
82 to 243 i.e. 3^5 -------------5
244 to 729 i.e. 3^6 ------------6
730 to 2187 i.e. 3^7----------- 7
2188 to 6561 i.e. 3^8 ----------8
hence for 6561 balls, u need to weigh 8 times.
in the case of a spring balance, take the powers of 2 instead of 3. answer would be 13 ( as 2^12=4096 n 2^13=8192 and 4096<6561<8192)
hope this helps.:smile: