Teach an equation and explain why Congcong said, "I've done 2.5 times as many mental arithmetic problems as Fangfang." Fangfang said, "if I do 18 more problems, I'll do as much as Congcong does." how many problems did they do?

Teach an equation and explain why Congcong said, "I've done 2.5 times as many mental arithmetic problems as Fangfang." Fangfang said, "if I do 18 more problems, I'll do as much as Congcong does." how many problems did they do?

Let x, y respectively
From "I've done 2.5 times as much mental arithmetic as Fangfang today."
x=2.5y
From "if I do 18 more questions, I'll do as much as Cong Cong does."
x=y+18
Substitute the first form into the second
2.5y=y+18
y=12
x=30

He forgot both of the_.
A rooms numbers
B room number
C room's number
D room numbers

Let's talk about it
After a long time, I have two people

To explain, but not meaningless irrigation
It is known that the length of a railway bridge is 500m, and there is a train passing through the bridge. It is measured that it takes 30s from the time when the train starts to get on the bridge to the time when the train completely crosses the bridge, and the time when the whole train is completely on the bridge is 20s. The speed of the train and the length of the train are calculated
When an aircraft flies between two cities, the wind speed is 24km ＼ h, it takes 2h50min to fly along the wind, and 30h to fly against the wind

The concept of 30 seconds is different from that of 20 seconds
1. The 30 second journey includes the length of the train
2. 20 seconds does not include the length of the train
Methods: ① 500 / 20 = 25 m / S (train speed)
30*25=750 m
750-500 = 250 (train length)
2. Solution. Let the velocity be x km / h without wind, then the velocity is (x + 20) km / h with wind and (x-24) km / h against wind, 17 / 6 (x + 24) = 3 (x-24)
17/6x+68=3x-72
140=1/6
x=840
The distance between the two places is (840-24) * 3 = 2448
A: the speed of the plane is 840 km / h and the distance is 2448 km