Question

A mattress store sells only king, queen and twin-size mattresses. Sales records at the store indicate that the number of queen-size mattresses sold is one-fourth the number of king and twin-size mattresses combined. Records also indicate that three times as many king-size mattresses are sold as twin-size mattresses. Calculate the probability that the next mattress sold is either king or queen-size.

Answer 1

Answer:

The probability that the next mattress sold is either king or queen-size is P=0.8.

Step-by-step explanation:

We have 3 types of matress: queen size (Q), king size (K) and twin size (T).

We will treat the probability as the proportion (or relative frequency) of sales of each type of matress.

We know that the number of queen-size mattresses sold is one-fourth the number of king and twin-size mattresses combined. This can be expressed as:

$P_Q=\dfrac{P_K+P_T}{4}\\\\\\4P_Q-P_K-P_T=0$

We also know that three times as many king-size mattresses are sold as twin-size mattresses. We can express that as:

$P_K=3P_T\\\\P_K-3P_T=0$

Finally, we know that the sum of probablities has to be 1, or 100%.

$P_Q+P_K+P_T=1$

We can solve this by sustitution:

$P_K=3P_T\\\\4P_Q=P_K+P_T=3P_T+P_T=4P_T\\\\P_Q=P_T\\\\\\P_Q+P_K+P_T=1\\\\P_T+3P_T+P_T=1\\\\5P_T=1\\\\P_T=0.2\\\\\\P_Q=P_T=0.2\\\\P_K=3P_T=3\cdot0.2=0.6$

Now we know the probabilities of each of the matress types.

The probability that the next matress sold is either king or queen-size is:

$P_K+P_Q=0.6+0.2=0.8$