Answer the following question, with explanation first.
Which of the following CANNOT be weighed using a balance scale and these unit weights: 2, 4, 7, and 10\nOptions:\n(A) 16\n(B) 15\n(C) 18\n(D) 19\n(E) 21

(B)
A.

13 (can be weighed by keeping weights of 10, 2 and 4 )\nB.

15 (cannot be weighed)\nC.

17 (can be weighed by keeping weights of 10 and 7 kg)\nD.

19(can be weighed by keeping weights of 10, 2 and 7 kg)\nE.

21 (can be weighed by keeping weights of 10, 7 and 4 kg).

So, the final answer is (B)