Question

Let X equal the number of hours that a randomly selected person from City A reads the news online each day. Suppose that X has a continuous uniform distribution on the interval [1, 4]. Let Y equal the number of hours that a randomly selected person from City B reads the news online each day. Suppose that Y has a continuous uniform distribution on the interval [1,5]. Assume that X and Y are independent. Determine P(Y> X).

Answer 1

Answer:

$P(Y>X) = \frac{17}{32}$

Step-by-step explanation:

Recall that since X is uniformly distributed over the set [1,4] we have that the pdf of X is given by $f(x) = \frac{1}{3}$ if $1\leq x \leq 4$ and 0 otherwise. In the same manner, the pdf of Y is given by $g(y) = \frac{1}{4}$ if $1\leq y\leq 5$ and 0 otherwise.

Note that if Y is in the interval (4,5] then Y>X by default. So, in this case we have that P(Y>X| y in (4,5]) = 1. We want to calculate the probability of having Y in that interval . That is

$P(Y>4) = \int_{4}^{5}\frac{1}{4}dy = \frac{1}{4}$. Thus, $P(Y\leq 4 ) = 1 - P(Y>4)= \frac{3}{4}$.

We want to proceed as follows. Using the total probability theorem, given two events A, B we have that

$P(A) = P(A|B)\cdot P(B) + P(A|B^c) \cdot P(B^c)$ In this case, A is the event that Y>X and B is the event that Y is in the interval (4,5].

If we assume that X and Y are independent, then we have that the joint pdf of X,Y is given by $h(x,y) = f(x)g(y) = \frac{1}{12}$ when $1\leq x \leq 4, 1\leq y \leq 4$. We can draw the region were Y>X and the function h(x,y) is different from 0. (The drawing is attached). This region is described as follows: $1\leq x \leq 4$ and $x\leq y \leq 4$, then (the specifics of the calculations of the integrals are ommitted)

$P(Y>X| y \in [1,4)) = \int_{1}^{4}\int_{x}^{4}\frac{1}{12}dy dx = \frac{1}{12}\int_{1}^{4} (4-x) dx = \frac{1}{12}\left.(4x-\frac{x}{2})\right|_{1}^{4}= \frac{1}{12}(4\cdot 4 - \frac{4^2}{2}-(4-\frac{1}{2}) = \frac{9}{2\cdot 12} = \frac{3}{8}$

Thus,

$P(Y>X) = 1\cdot \frac{1}{4} + \frac{3}{8}\cdot \frac{3}{4} = \frac{17}{32}$