| Exam Board | WJEC |
|---|---|
| Module | Unit 4 (Unit 4) |
| Year | 2019 |
| Session | June |
| Marks | 5 |
| Paper | Download PDF ↗ |
| Mark scheme | Download PDF ↗ |
| Topic | Conditional Probability |
| Type | Standard Bayes with discrete events |
| Difficulty | Moderate -0.8 This is a straightforward conditional probability question using the law of total probability and Bayes' theorem. Part (a) requires calculating P(works) = Σ P(works|supplier) × P(supplier) with given values, and part (b) applies Bayes' theorem directly. The arithmetic is simple, and both parts follow standard S1/S2 procedures with no conceptual challenges or problem-solving required. |
| Spec | 2.03c Conditional probability: using diagrams/tables2.03d Calculate conditional probability: from first principles |
Val buys electrical components from one of 3 suppliers $A$, $B$, $C$, in the ratio $2:1:7$. The probability that the component is faulty is $0.33$ for $A$, $0.45$ for $B$ and $0.05$ for $C$. Val selects a component at random.
\begin{enumerate}[label=(\alph*)]
\item Find the probability that the component works. [3]
\item Given that the component works, find the probability that Val bought the component from supplier $B$. [2]
\end{enumerate}
\hfill \mbox{\textit{WJEC Unit 4 2019 Q1 [5]}}