Analysis: let the ship speed be x and the water speed be y.
1/(x+y)=7 x+y= 1/7 ①
1/(x-y)=8 x-y= 1/8 ②
①+②
2x =15/56x =15/112 substituted in ①.
15/ 1 12+y = 1/7y = 1/ 1 12
A group of soldiers set out from the camp to the shooting range at a speed of 5 kilometers per hour. After leaving 10km, the correspondent went back for certification and then returned to the pursuer team. His speed is 9 kilometers per hour. When he caught up with the team, the team walked (25)km.
Analysis: let the correspondent's round-trip time be X.
Travel distance of correspondent: return to camp 10km, return to starting point 10km, and catch up with troops 5x.
9x=5x+ 10.2，x = 5。
The team left: 5.5 = 25.
3. A train is 60 meters long and the speed is 36 kilometers per hour. There is a car with a length of 10 meter in front of the train, and the speed of the train is 18 kilometers per hour, which exceeds the needs of the car for (40) seconds. ..
Analysis: If the train wants to completely overtake the car, it needs the tail of the train to reach the front of the car.
This distance is the distance that the train needs to catch up with = 10+ 130+60.
18km/h = 5m/s 36km/h =10m/s.
It takes x seconds for a train to overtake a car.
10x = 5x+ 10+ 130+60，x=40。
4. Equation 3mx+n=(4m-2)x+3
When m=(2), n=(3)x has countless solutions.
When m=(2), n≦(3)x has no solution.
When m≦(2), x=((n-3)/(m-2)) is the only solution.
5. When Party A is Party B's current age, Party B is 8 years old; When Party B is Party A's current age, Party A is 29 years old and Party A is (7) years older than Party B. ..
Analysis: Suppose B's age is X now.
Jiayi
When x 8 A is B's current age
29 29-x-8 B is your current age (the age difference between A and B will not change).
29-x-8-x=x-8 solution: x= 15.
A is bigger than B: x-8=7.
6. In the promotion activities of a shopping mall, there is no discount for shopping below 300 yuan, and 10% discount for shopping below 300 yuan and 600 yuan. 600 yuan offers a discount of 10%, which is 20% lower than that of 600 yuan. If someone buys it twice, they are 120 yuan and 580 yuan, if he buys it once, it will save (70) yuan, and if he buys it once, it will save (140) yuan.
Analysis:
Two shopping discounts: 120.9+580.9 = 630.
One-time shopping discount: (120+580) 0.8 = 560
No discount: 120+580=700