The first question: if you answer the X question correctly, you will answer the (50-X) question incorrectly. The countable inequality is 4X-(50-X)≥ 120.

Solution: X≥34, so Xiao Ming must answer at least 34 questions correctly.

Question 2: If you have x 10 yuan banknotes, you have (60-X) 50 yuan banknotes. The countable inequality is 10x+50 (60-x) < 200. The solution is: X is greater than 70, then Xiaoying has at least 70 pieces 10 yuan.

Question 3: Suppose I walked X kilometers this month. ① If100+0.1x >1.35x gets x < 80, then it is economical for me to rent a car in Xiao Zhao for a trip within 80km. ② If100+0.1x =1.35x gets x = 80, it is also cost-effective to rent a car from Xiao Wang or Xiao Zhao when the driving distance is equal to 80km. ③ If100+0.1x <1.35x gets x > 80, it is more economical to rent Xiao Wang's car when driving more than 80km.

Question 4: Suppose the bus travels at least X kilometers per hour. The countable inequality is: 3000x (times) (5/6) +45X≥ 10000 solution: X≥39 A: the bus travels at least 39 km/h.

Question 5: If you answer question X correctly, the answer will be (29-X), that is, (30- 1-X). The countable inequality is 6x-2 (29-x) > 85. The solution is: X > 17.875 because x must be an integer, X > 18 A: he got it right at least 18.