How to calculate probability question 3 million out of 18 million users could not use their devices as cellphones and that 1 million could not use their devices as a cellphone and for data device.what is the probability that a randomly chosen device could not be used either for data or for voice communication?
My basic approach is by using the below formula $( P(A \cup B) =P(A)+P(B) -P( A \cap B)$
$P(A \cup B)= P(A) +P(B)- P(A \cap B) = 3/18 + P(B) – 1/18$.
Can someone please guide as above equation has two unknowns. I am confused. 
Answer is showing 0.3889
 A: From my perspective, the randomly chosen device that could not be used either for data or for voice communication means that the device cannot be used for data solely or cannot be used for voice communication solely, but not both at the same time.
To start with, the given probability of not using for data = $\frac {3}{18}$.
However, this event may include an event that the device is not used for both data and voice communication at the same time. 
$$P(not\ for\ data)=P(not\ for\ data\ solely) + P(not\ for\ data \cap not\ for\ voice\ communication)$$
$$P(not\ for\ data\ solely)=P(not\ for\ data)-P(not\ for\ data \cap not\ for\ voice\ communication)$$
$$P(not\ for\ data\ solely)=\frac {3}{18} - \frac{1}{18} = \frac{2}{18}$$
On the other hand, assuming the events that a device is not used for voice communication and data are independent, 
$$P(not\ for\ data \cap not\ for\ voice\ communication)=P(not\ for\ data) \times P(not\ for\ voice\ communication)$$
$$P(not\ for\ voice\ communication)=\frac {1}{18} ÷ \frac {3}{18} = \frac {1}{3}$$
Likewise, this event may include an event that the device is not used for both data and voice communication at the same time. Hence,
$$P(not\ for\ voice\ communication)=P(not\ for\ voice\ communication\ solely) + P(not\ for\ data \cap not\ for\ voice\ communication)$$
$$P(not\ for\ voice\ communication\ solely)=P(not\ for\ voice\ communication)-P(not\ for\ data \cap not\ for\ voice\ communication)$$
$$P(not\ for\ voice\ communication\ solely)=\frac {1}{3} - \frac{1}{18} = \frac{5}{18}$$
Finally, adding up the probability of not using for data solely and voice communication solely = $\frac {2}{18} + \frac {5}{18} = \frac {7}{18} \approx 0.3889$ (corr. to 4 d.p.) 
