That is, the classification of age difference constant questions: sum difference age, sum difference age and indirect age difference questions. ?

1 and three characteristics of age:

The age difference between them is constant; Their age increases or decreases simultaneously; The multiples of their age are changing.

Step 2: Example

(1) Zhuang Zhuang is 9 years old and Yue Yue 12 years old. How old were they when they were 39?

Method 1: the age difference is constant and solved according to the sum difference problem.

① Age difference: 12-9=3 (years old) ② Yue Yue: (39+3)÷2=2 1 (years old) ③ Zhuang Zhuang: 2 1-3 = 18 (years old).

A: When they are 39 years old, Zhuang Zhuang is 18 years old and Yue Yue is 2 1 year old.

Method 2: Age increases with age.

① Total age of this year: 12+9=2 1 (year) ② How many years have passed: (39-2 1)÷2=9 (year).

③ Zhuang: 9+9= 18 (years old) ④ Yue Yue: 12+9=2 1 (years old).

A: When they are 45 years old, they are 18 years old and Yue Yue is 2 1 year old.

(2) Zhuang Zhuang and his father are both 48 years old, and his father's age is just five times that of Zhuang Zhuang. In a few years, his father will be three times as old as a strong man. A few years later, my father was twice as old as a strong man.

Analysis: (1) Using the basic formula of the sum multiple problem and ÷ (multiple+1)= decimal, we can find that this year's strong age is 48÷(5+ 1)=8 (years old), and the father's age is 48-8=40 (. The age difference between them is: 40-8=32 (years)

Because of the same age difference, when dad's age is three times that of Zhuang Zhuang, Zhuang Zhuang's age is 32÷(3- 1)= 16 (year), so it is 16-8=8 (year).

(2) When dad is twice as old as Zhuang Zhuang Zhuang, Zhuang Zhuang's age is 32÷(2- 1)=32 (years).

So, 32-8=24 years later.

Summarize and remember

1, formula summary

(1) age difference ÷ multiple difference = age (age when multiple relation is satisfied at that time)

(2) After several years, age = age difference ÷ multiple difference-young age.

(3) Age a few years ago = young age-age difference ÷ multiple difference.

Step 2 remember

"Age problem" is a typical problem in the first volume of mathematics in the third grade of primary school. When solving this kind of problems, we should keep in mind: to solve the problem of "how many times is a number", we must use multiplication to calculate. Find how many times one number is another, that is, how many other numbers are there in a number, and calculate by division.