The random variable X X\,X has the following probability distribution.
| xxx | 1 | 2 | 3 | 4 | 5 |
| P(X=x)P(X=x)P(X=x) | kkk | k2\displaystyle \frac{k}{2}2k | k3\displaystyle \frac{k}{3}3k | k4\displaystyle \frac{k}{4}4k | k6\displaystyle \frac{k}{6}6k |
where k k\,k is a constant. You may use k=49\displaystyle k=\frac49k=94.
The random variables X1 X_1\,X1 and X2 X_2\,X2 are independent and each have the same distribution as XXX. Find P(X1+X2=7)P(X_1 + X_2 = 7)P(X1+X2=7)
215 exam-style questions on Edexcel A Level Maths Statistical Distributions, covering 6.1 Probability Distributions, 6.2 The Binomial Distribution, and 6.3 Cumulative Probabilities. Each one has a worked solution and a mark scheme showing where the marks go.