Question

Jeff 旋转转盘 $P$ 、 $Q$ 和 $R$ ，并把得到的数字相加。他的和为奇数的概率是多少？

Jeff rotates spinners $P,$ $Q$ and $R$ and adds the resulting numbers. What is the probability that his sum is an odd number?

$\dfrac{1}{4}$

$\dfrac{1}{3}$

$\dfrac{1}{2}$

$\dfrac{2}{3}$

$\dfrac{3}{4}$

转盘 $Q$ 总是落在偶数上，所以不改变总和的奇偶性。转盘 $R$ 总是落在奇数上。

为了让总和为奇数，转盘 $P$ 的数必须是偶数。转盘 $P$ 的三个等可能区域中只有一个是偶数。

因此概率为 $\dfrac{1}{3}$ 。

所以正确答案是 B。

Spinner $Q$ always lands on an even number, so it does not change the parity of the total. Spinner $R$ always lands on an odd number.

For the full sum to be odd, the number from spinner $P$ must be even. Only one of the three equal sectors of spinner $P$ contains an even number.

Therefore the probability is $\dfrac{1}{3}$ .

Thus, B is the correct answer.