The formula for calculating the expected number of people is
Ex = xPx
where
Px represents the probability of an event, x
From the information given,
when it is raining, 6000 people will attend
Probability that it will rain = 40% = 40/100 = 0.4
when it is not raining, 16,000 people will attend
Probability that it will not rain = 1 - 0.4 = 0.6
Thus,
Expected number of people who will attend this concert = (6000 * 0.4) + (16000 * 0.6) = 2400 + 9600
Expected number of people who will attend this concert = 12000