Is the contrapositive of "If P is true, then Q is true" "If Q is not true, then P is not true"?

 Yes, the contrapositive of "If 

$P$ is true, then $Q$ is true" is indeed "If $Q$ is not true, then $P$ is not true."

Here's why:

The statement "If $P$, then $Q$" can be written in logical form as:

$$P \rightarrow Q$$

The contrapositive of a conditional statement $P \rightarrow Q$ is the statement obtained by negating both the hypothesis and the conclusion, and then reversing the direction of the implication. That is:

$$\neg Q \rightarrow \neg P$$

So, the contrapositive of "If $P$ is true, then $Q$ is true" (i.e., $P \rightarrow Q$) is "If $Q$ is not true, then $P$ is not true" (i.e., $\neg Q \rightarrow \neg P$).

In logic, the original statement and its contrapositive are logically equivalent, meaning they have the same truth value.