You have 8 balls. one of them is defective and weighs less than others. use a balance to measure balls against

Question

each other. doing this twice. how do you find the defective one

Answer 1

I assume that the balls are numbered.

step 1)  choose  ball 1 and 2 and put them aside

weight balls 3 4 5 against 6 7 8

if equal, then check 1 against 2, and we are done.

step 2)  let's say that 3 4 5 are found lighter.

we now know that 6 7 and 8 are normal.

put 3 aside

put 4 with 6

put 5 with 7

weight 4 6 against 5 7.

If equal then it is ball 3,

if 4 6 is lighter then it is ball 4

if 5 7 is lighter then it is ball 5.

Answer 2

The answer by MathJakk is correct and will give you the result.  However, in step 2 it is not necessary to combine and weigh ball 4 and 6 against 5 and 7.  Since 345 are found lighter we now know that 678 are normal.  Simply concentrate on 345. Put 3 aside and simply weigh 4 against 5.  If 4 and 5 are equal then the defective ball is 3.  If 4 or 5 is lighter then the defective ball is the lighter of the two.