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It is known that the speed ratio of the bus to the truck is 5 / 4. How many hours does it take for the truck to complete the whole journey?
Consider the total distance as 1
The speed ratio of passenger cars to freight cars is 5 / 4
The speed of passenger car and freight car is 5x and 4x respectively
4（5X+4X）=1
X=1/36
The speed of the truck is one ninth
It takes 1 / (1 / 9) = 9 hours for a truck to complete the whole journey
When the two cities meet for 160 km, a ship will return immediately after sailing from city a to city B. it will go for 9 hours. If the speed of the current is 4 km / h, the speed of the ship in still water can be calculated. Let the speed of the ship in still water be x km / h. according to the meaning of the question, the equation is?
160/(X+4)+160/(X-4)=9
When an aircraft flies over the anti-aircraft gun position, the probability of each anti-aircraft gun hitting the aircraft is 0.4. How many guns should be deployed on the position to make the probability of high artillery hitting the aircraft reach 95%?
When the probability of miss is less than 5%, it meets the requirement
1-0.4=0.6
0.6 * 0.6 * 0.6 * 0.6 * 0.6 = 0.046656, less than 0.05
So we need five antiaircraft guns
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