How many different ways are there to divide 20 by the sum of three different natural numbers not greater than 9? Please list them separately.
[Answer analysis]
20=9+8+320=9+7+420=9+6+520=8+7+5.
extreme
□+□+□+□+□=30
Fill in five consecutive natural numbers in the above □ to make the equation hold.
[Answer analysis]
4+5+6+7+8=30
Tisso
Move 1 matchsticks in the following categories to make each category the correct formula.
Answer:
1, (move a 1 from 1 to1to become 7,7+7 =14)
2. (Move 1 in 2 1 to the minus sign, 2+2=4)
Article 4
Aunt gives candy to the children. If you give each child 3 yuan, you will get 2 yuan more. If each child gives 4 yuan, there will be 4 yuan less. Aunt has a piece of candy, which should be given to children.
Answer: 2+4=6 (block) 3×6+2=20 (block)
chapter five
? 1, starting from 2 in the lower left corner, fill in "+"or "-"between the numbers so that the final result is equal to 7.
2 4 6 9 5 1 = 7
2. The small conference room of the school has four seats in the first row, with two more seats in each row than in the previous row, and the last row has 18 seats. How many seats are there in this conference room?
Answer analysis:
1, starting from 2 in the lower left corner, fill in "+"or "-"between the numbers so that the final result is equal to 7.
2 4 6 9 5 1=7
2 + 4 + 6 – 9 + 5 – 1 = 7
2. The small conference room of the school has four seats in the first row, with two more seats in each row than in the previous row, and the last row has 18 seats. How many seats are there in this conference room?
(18-4) ÷ 2+ 1 = 8 (row)
(18+4)×8÷2=88 (pieces)
Article 6
? 1、 5 5 5 5 5= 10
9 9 9 9 9= 10
2. At the party, 10 fruit should be packed in 6 bags. It is required that the fruits in each bag should be even, and there should be no fruits and bags left. How should I install it?
Answer analysis:
1、 5 5 5 5 5= 10 (5×5-5-5-5= 10)
9 9 9 9 9= 10 (9—9+9÷9+9= 10)
2. At the party, 10 fruit should be packed in 6 bags. It is required that the fruits in each bag should be even, and there should be no fruits and bags left. How should I install it?
Put 2 bags in each bag, and then put 5 bags in the last bag. )
Article 7
1, 10, 20, 30, 40, ... The 90th item in this series is ().
2. There are 10 locks whose keys are messed up. How many times should I try to get each lock to have its own key?
Answer analysis:
1, 10, 20, 30, 40, ... The 90th item in this series is ().
Answer:10+(90-1) ×10 = 900.
2. There are 10 locks whose keys are messed up. How many times should I try to get each lock to have its own key?
Answer: 9+8+7+6+5+4+3+2+ 1=45.