You have a balance scale and wish to verify the weights of items that come in weights from 1 to 121 (all integral). What is the minimum number of fixed weights you need?
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W e i g h t u s e d a r e 1 , 3 . . . . . . . . . . 1 , 3 − 1 = 2 , 3 , 1 + 3 = 4 . T o g e t 5 , w e s h o u l d h a v e X , s o X − 4 = 5 . S o X = 9 . W i t h 1 , 3 , 9 . . . . . . . w e g e t 9 − 4 = 5 , 9 − 3 = 6 , 9 + 1 − 3 = 7 , 9 − 1 = 8 , 9 , 9 + 1 = 1 0 , 9 + 3 − 1 = 1 1 , 9 + 3 = 1 2 , 9 + 3 + 1 = 1 3 . T o g e t 1 4 , w e s h o u l d h a v e Y , s o Y − 1 3 = 1 4 . S o Y = 2 7 . W i t h 1 , 3 , 9 , 2 7 w e c a n g e t u p t o 1 3 + 2 7 = 4 0 a s a b o v e . T o g e t 4 1 , w e s h o u l d h a v e Z , s o Z − 4 0 = 4 1 . S o Z = 8 1 . W i t h 1 , 3 , 9 , 2 7 , 8 1 w e c a n g e t u p t o 4 0 + 8 1 = 1 2 1 a s a b o v e . S o t h e r e a r e o n l y F I V E w e i g h t s r e q u i r e d .