36-30 A person can travel to and from work by train or by car. If he takes the train in the morning, he takes the car in the afternoon; and if he takes the train in the afternoon, he takes the car in the morning. In $x$ days, this person takes the train 9 times, the car in the morning 8 times, and the car in the afternoon 15 times. The value of $x$ is
(A) 19 .
(B) 18 .
(C) 17 .
(D) 16 .
(E) The given information is insufficient to solve the problem.
(26th American High School Mathematics Examination, 1975)
[Solution] According to the problem, the total number of rides this person took is $2x$, so $2x = 9 + (8 + 15)$, solving for $x$ gives $x = 16$.
Therefore, the answer is $(D)$.