Which of the following CANNOT be weighed using a balance scale and these unit weights: 1, 4, 7, and 10	Pick one.
A) 13  B) 15  C) 17  D) 20  E) 21
A. 18 (can be weighed by keeping weights of 10,1 and 7 )   B. 15 (can be weighed by keeping weights of 10,4 and 1 kg)   C. 17 (can be weighed by keeping weights of 10 and 7 kg)   D. 20(cannot be weighed)   E. 21 (can be weighed by keeping weights of 10, 7 and 4 kg)   20
<result>D</result>
Which of the following CANNOT be weighed using a balance scale and these unit weights: 1, 4, 7, and 10