Article 20

P 10:

1, Wang Xin's final exam results are as follows: the average score of Chinese mathematics is 94, the average score of mathematics foreign language is 88, and the average score of foreign language is 86. How many points did Wang Xin get in Chinese, maths and foreign languages?

Thinking of solving problems Chinese+math+foreign language =94+88+86=268 points, 268-94? 2=80 points, that is, foreign language scores; 268-88? 2=92 points, which is a language achievement; 268-86? 2=96 points, which is a math score.

2. As shown in the table below, the words and numbers in each column form a group. For example, what is 1 group? Do 2？ And the second group is? O 0？ ... then the 25th group is ().

Run the Olympics, run the Olympics, run the Olympics. ......

2 0 0 8 2 0 0 8 2 0 ......

What are the ideas for solving each group of problems? Words? And then what? Count? Composition, words? Is it a trio or a group of 25? 3=8 groups ... 1,? Words? For what? Do it. ; ? Count? Is it a foursome or a twentyfive? 4=6 groups ... 1,? Count? For what? 2? And what is the 25th group? Do 2？ .

P 12:

1, there is a column number 2, 5, 8, 1 1, 14 ... What is 104 in this column number?

The 1 number of this series is 2. Starting from the second number, it is all obtained by adding 3 to the previous number. 104-2= 102. What you need is how much to add to the first number (2) to get 104. 102? 3=34, add 34 3 to the first number to get104; 34+ 1=35, 104 is the 35th position in the series.

P20:

There is a frog at the bottom of the well. It is understood that the depth of this well is 25 meters. Frogs jump 3 meters up and fall 1 meter every day. How many days can a frog jump out of the well?

The frog's last day was special. It jumped out of the well after jumping 3 meters and did not slide down. 25-3=22 meters, the remaining 22 meters jump 3 meters per day 1 meter, and the actual jump is 3- 1=2 meters; 22? 2= 1 1 day, that is, 22 meters need to jump 1 1 day, the last 3 meters need 1 day,1=12 days, that is, frogs jump.

3. A school arranges student dormitories. If there are 5 people in each room, there will be 14 people without beds. If there are 7 people in each room, will there be 4 extra beds? So, how many dormitories and students are there?

The idea of solving problems is profit and loss problem in the Olympic Mathematics, which often appears in the fourth grade problems. As with the above topic, it is known that there are two distribution schemes, one is surplus, the other is deficiency, and the number of participants and the total amount of distribution are sought. This topic is called profit and loss problem (the profit is called profit, and the loss is called loss). To solve the problem of profit and loss problem, we often use the comparison method to find out the total difference between the two distribution results and the difference between the two distribution numbers, first find out the number of people participating in the distribution, and then find out the total amount of distribution. The formula is: total difference? Difference per copy = number of copies.

The difference between the first time and the second time 14+4= 18 people; At two o'clock, the difference between each dormitory is 7-5=2 people; 18? 2=9 rooms, ask for 18 dormitory; 9? 5+ 14=59 people, and ask for the number of students.

P24:

2. Seven apples should be distributed equally to 12 children, so that each child can get two pieces. How should I divide it?

Problem solving ideas 12 children, each child is divided into two pieces. From the above two conditions, it is concluded that * * * needs 24 apples. You can divide three apples into four pieces to get 12 pieces, and give one piece to each person first; Then divide the remaining four apples into three pieces evenly, and get 12 pieces, one for each person, so that each child is divided into two pieces.

P28:

1, Xiao Ming asked Miss Li how old she is this year, and Miss Li said: When I was your age, you were only 3 years old; I was 42 years old when you were my age. ? Do you know how old Miss Li is this year?

The way to solve the problem is the problem of age. To solve the age problem, we must grasp it? The age difference remains the same? This method, (42-3)? 3= 13 years old, which is the age difference between Miss Li and Xiao Ming; 3+ 13= 16 years old, which is Xiaoming's age this year; 16+ 13=29 years old, which is Miss Li's age this year. This question can be drawn to help you understand.

P30:

3. Five students, A, B, C, D and Xiao Ming, play chess together. Every two students must play a game. So far, A has played 4 sets, B has played 3 sets, C has played 2 sets, and D has played 1 set. How many sets did Xiao Ming play?

The topic of problem-solving ideas is closely related to the arrangement and combination of last semester in grade three, and can be drawn to help understand. A played four games, which means A played one game against B, C, D and Xiao Ming respectively. B played three games and C played two. Through these two conditions, B must have played a game with A and C. For the time being, it is uncertain who B will play with Ding and Xiao Ming, while C will play with A and B respectively. The last condition is that Ding plays 1, and Ding's game must be with A, not with others, so it can be determined that the third game of B is with Xiao Ming; Based on the previous speculations, it is concluded that Xiao Ming played two sets, one against A and the other against B.

P32:

2. Uncle Zhang's office is at the Blue Sky Building18th floor. One day, due to the power failure, he had to walk upstairs. He calculated that it took 100 second from the first floor to the sixth floor. At this rate, how long will it take to get to the18th floor?

The key to solve this problem is to find the upper level. The time required is 100? (6- 1)=20 seconds; 18-6= 12 floor. What I want is for Uncle Zhang to walk from the 6th floor to18th floor and12nd floor,12nd floor? 20=240 seconds, it takes time.

P38:

1. There is a bus with 10 stop from the start to the end. One day, the car drove from the starting point to the end point. 1 Station has 9 passengers, and the second station has 8 passengers. In the future, more passengers will get off at each stop than at the last stop 1, and fewer passengers will get on at each stop 1. How many seats does this car need to have so that every passenger can have a seat?

Combining numbers with shapes, the idea of solving problems is relatively simple and easy to understand. First stop: 9 people; Second stop: 9- 1+8= 16 people; Third stop: 16-2+7=2 1 person; Fourth stop: 2 1-3+6=24 people; Fifth stop: 24-4+5=25 people. There are more people getting off from the sixth stop than getting on, and the total number of people on the bus is gradually decreasing, so this car should have at least 25 seats.

P40:

There are two baskets of apples. Basket a weighs 85 kilograms, and basket b weighs 35 kilograms. How many kilograms do you need to take out from basket A and put it into basket B, so that the weight of basket A is twice that of basket B?

In this problem, the total mass of two baskets of apples has not changed, which is 85+35 =120kg; To make the mass of basket A twice that of basket B, then basket B should have 120? (2+ 1)= 40k g； Basket B used to have 35 kilograms, but now it has 40 kilograms, which means that basket B needs to be given 40-35 = 5 kilograms from basket A. ..

P42:

1, A, B, C and D arrange seats. At first, A sits at 1, B sits at 2, C sits at 3 and D sits at 4. Then they kept switching places, the first time was the upper and lower rows, the second time was the left and right columns after the first exchange, the third time was the upper and lower rows, and the fourth time was the left and right columns, and so on. Q: After the tenth exchange of places, where did C sit?