Let the train length be x, then the train will travel 1680+x meter140s, and travel 1680-x meter100s.
( 1680+x)/ 140 =( 1680-x)/ 100
5( 1680+x)=7( 1680-x)
8400+5x= 1 1760-7x
12x=3360
x=2800
That is, the length of the train is 280 meters.
Speed: (1680-280)/100 =14m/s.
2. The total length of railway bridge is1200m. It takes 75 seconds for the train to cross the bridge and 15s for the train to cross the roadside pole. Find the length of the train.
Train route 15 second is the total length of a train.
Then 75 seconds is the total length of five trains.
75 seconds is1200m plus the full length of a train.
Then the total length of a train is: 1200/[5- 1] = 300m.
The father is 65 years old and the son is 30 years old. How many years ago my father was eight times older than my son?
Father is older than son.
65-30 = 35
A few years ago, my father was older than my son.
8- 1 = 7 times
The age of the son at this time is
35 ÷ 7 = 5 years old
A few years ago, my father was eight times as old as my son.
30-5 = 25 years ago
The student asked the teacher how old he was this year. The teacher said, "When I was your age, you were only 3 years old. When you were my age, I was 39 years old." How old is the teacher this year?
Let the students be x years old.
The teacher is X years old and the student is 3 years old, so the teacher is x-3 years older than the student.
Because students are always x-3 years younger than teachers.
So when the teacher is 39, the student is 39-(x-3), which is the age of the teacher now.
The teacher's present age is also the student's present age X plus the age difference with the teacher x+x-3.
So the teacher's age =39-(x-3)=x+x-3 x= 15, that is, the current students are 15 years old.
Then the teacher is x-3= 12 years older than the students.
So the teacher is 27 years old.