An eccentric casino owner decides that his casino should only use chips in $3 and $7 denominations. Which of the following amount cannot be paid out using these chips?  Answers
A) $27
B) $24
C) $23
D) $25
E) $21
Yes, the number of 3 or/and 7 dollar chips can be zero, however both being zero mean that the casino is paying out $0, which is not realistic.   Each option but D can be represented as the sum of a multiple of 3 and a multiple of 7:   A. $27 = 7*3 + 3*2   B. $23 = 7*2 + 3*3   C. $24 = 7*3 + 3*1   E. $21 = 7*3   Answer: $25.
<result>D</result>
An eccentric casino owner decides that his casino should only use chips in $3 and $7 denominations. Which of the following amount cannot be paid out using these chips?