Question

In a cafe, a customer orders one drink. The probability that he orders tea is 0.42. The probability that he orders coffee is 0.3. Work out the probability that he orders either tea or coffee

Answer 1

For any given set of probabilities, the total is always 100% or 1.0. (i.e. We know he ordered SOMETHING, so all the choices have to be accounted for.)

$P_{total} = 1.0 = P_{tea} + P_{coffee} + P_{neither}$
$1.0=0.42+0.3+P_{neither}$
$P_{neither}= 1.0 - 0.72 = 0.28$