The probability of not seeing a supercar in 20 minutes is:
= 1 − P(Seeing one supercar)
= 1 − 0.3
= 0.7
Probability of not seeing any supercar in the period of 60 minutes is:
= (0.7) ^ 3 = 0.343
Hence, the probability of seeing at least one supercar in 60 minutes is:
= 1 − P(Not seeing any supercar)
= 1 − 0.343 = 0.657