The rate $2/3$ per minute means $2/3\times 60 = 40$ vehicles per hour. The random selection of the car types means that we may divide the Poisson process to two - with 10% = 4 trucks per hour and 90% = 36 cars per hour.
a) The probability that no truck passes in some time is exponentially decreasing with the rate indicated above, so after one hour it is $\exp(-4)$. The probability that at least one truck passes is the complement, $1-\exp(-4)=98.16$ percent.
b) The expected number of vehicles is the expected number of trucks plus the expected number of cars. The former is known to be 10. The latter is independent and it is 36, as explained in the first paragraph. So the total is 46.
c) This is harder. The probability that 50 vehicles passed - something we know that occurred - is $40^{50} \exp(-40)/50! = 1.77$ percent. The probability of a more refined event - subset of the previous one - that it was 5 trucks and 45 cars is $4^{5} \exp(-4)/5! \times 36^{45} \exp(-36)/45! = 0.156 \times 0.021 = 0.00327 $ Now just divide $0.327$ percent by $1.77$ percent and you get $18.5$ percent.
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