here are the answer for the 8 block and balanced scale problem 

1.place three blocks on both sides of the balance scale

and then see the balanced scale

## if it balances

2a.that means no defect in that six blocks,because they have the same weight 

remove them all then place the other two, one on each side

and then see the balanced scale 

the light one is the defect block 

##if it doesn't balanced

2b. it means the defect block is  in the light side .only left three more

 Remove all the blocks from the heavy side, 

place one each from the three left on the light side on each side of the balance and keep the other one in your hand. If it balances the one in your hand is the defect. If it does not obviously, the lighter one is defect