Elementary School to Junior High School Placement Review Materials The exam is coming up tomorrow, p

If the son is a year old 12 years ago, the father is 10 years old.

Now the son is A+12 years old, and the father is 10A+12 years old.

After 12 years, the son is a + 12 + 12 years old, and the father is 10a + 12 + 12 years old. That is, (a+24)x2=10a+24, and the solution is a=3.

So my father is now 3x10+12=42 years old.

It's basically equation solving, and you don't need equations.

The age difference between the father and the son is a fixed value, note that 12 years ago the father's age was 10 times that of the son, 12 years later the father's age is twice that of the son, 12 + 12 = 24 years have passed in the middle, and the son's age 12 years ago was 24 (10-2) = 3 years old, then the father's age 12 years ago was 3 10 = 30 years old, and the father's current age is 30 + 12 = 42 years.

Answer: Remember an equivalence relationship for age, that is, no matter how many years ago and how many years later, the age difference is constant.

Let's say the father is now x years old. Then 12 years ago the father was x-12 years old. 12 years ago the son was (x-12) 10.

After 12 years, the father is x+12 years old, and the son is (x+12) 2 years old.

x-12-（x-12）/10=x+12-（x+12）/224+（x-12）/10-（x+12）/2=0240+x-12-5x-60=0

4x=168

x=42, so my father's current age is 42 years old.

<> elementary school should have learned equations, and then**The main points in it should be clearly marked, maybe the words are a little ugly, don't mind.

Suppose the age of the son is x, 10*x-12=2*x+128x=24 x=3

12 years ago, my son was 3 years old and my father was 30 years old. Now the son is 15 years old and the father is 42 years old. After 12 years, the son was 27 years old and the father was 54 years old.

So my father's current age is 42 years old.

Solution: Suppose 12 years ago, the son's age was x, and the father's age was 10x. 10x+24=2(x+24 ) The solution gives x equal to 3, and the father's age is 10 3+24=54. (As it should be).

Now my father is 42 years old. The son is 15 years old. I think it's a common sense reasoning question, and it is impossible to reason with the age of my son 12 years ago, 1 year old, 2 years old, 4 years old, 5 years old, and only 3 years old is eligible.

Each workshop draws out 3 stevedores, a total of 3 5 = 15 people, there are 3 people on each car, a total of 3 3 = 9 people, so that 15-9 = 6 (people) can be saved, at this time A has 3 people, B has 2 people, C has 4 people, D has 0 people, E has 5 people and then 2 people from A, B, C, E 2 people each, 2 people on each car, so that 2 4-2 3 = 2 people can be saved so that each car with 5 people, a total of 15 people, A has 1 person, B has 0 people, C has 2 people, E has 3 people, D or 0 people A total of stevedores:

5 3 + 1 + 2 + 3 = 21 (person).

It is written from 1 to 21 and erased is 15

The average is that it should add up from 1 21, just think about it. Then, add 1 to 21 to be 231, subtract a number, and the average of 20 numbers is, and the sum is 216, so the missing number is 231-216=15.

Assuming there are n numbers in total, then the erased number is n(n+1).

And because n is an integer n is 21, the erased number is 15