Selling Horses

Once upon a time, there was a merchant who was particularly shrewd. Once he bought a horse at the horse market for 10 taels of silver, and sold it for 20 taels of silver as soon as he changed hands; then he bought it in for 30 taels, and finally sold it for 40 taels. How much money did he make in this horse transaction?

Reference Answer:

This sale can be viewed in two ways. The first time he bought 10 taels of silver and sold it for 20 taels of silver, so he made 10 taels of silver. The second time he bought 30 taels of silver and sold 40 taels of silver, so he also made 10 taels of silver. In the horse transaction, the merchant **** made 20 taels of silver.

Number of People

Xiao Liang walked into the classroom and saw that there were only 8 students in the classroom, so how many students are there in the classroom a*** now?

Answer:

Careless kids will think that there are 8 students in the classroom, but this answer is wrong. After reviewing the question carefully, you can find that the question has already pointed out that "Xiaoliang walked into the classroom", so the number of students should include Xiaoliang, and there are 9 students in one ****.

A snail climbs up a 10-meter-deep well

A snail climbs up a 10-meter-deep well, 5 meters during the day, and then slides down 3 meters at night.

Answer:

The snail climbed up 5 meters during the day, and then fell down 3 meters at night, so in reality, it can only climb up 2 meters per day, and it took the snail 3 days to climb the first 6 meters, and there are still 4 meters left, so the snail will be able to climb out on the 4th day.

Race

The animals held animal games, in the long-distance race there are 4 animals running in front of the squirrels, there are 3 animals running behind the squirrels, a **** how many animals to participate in the long-distance race?

Reference Answer:

This question to clarify the key to the problem, we can run all the small animals as a queue, squirrels in front of 4 small animals, behind 3 small animals, in this queue, that is, there is no counting the squirrels themselves, so the total number of the team should also be added to the small squirrels. 4 + 3 +1 = 8 (only), a **** there are 8 animals to participate in the long-distance running race. The total number of squirrels in the team is added to the total number of squirrels.

Counting radishes

The white rabbit has 10 radishes. If the white rabbit gives the gray rabbit 3 radishes, they will both have the same number of radishes.

Answer:

If the white rabbit gives 3 radishes to the gray rabbit, both of them will have the same number of radishes, and if they have the same number of radishes, both of them will have the same number of radishes, how many radishes does the white rabbit have?

Natural series of interesting questions

This exercise, most of the natural series of counting problems, solve the problem of thinking is generally the use of enumeration and classification of statistical methods, hope that students can master it well.

Example 1 Xiaoming wrote from 1 to 100, he **** wrote how many numbers "1"?

Solution: Classification:

"1" appears in the single digit of the number:

1, 11, 21, 31, 41, 51, 61, 71, 81, 91*** 10;

"1 " appears in the tens place:

10, 11, 12, 13, 14, 15, 16, 17, 18, 19*** 10;

"1" appears in the hundreds place: 100*** 1;

*** counts 10+10+1=21.

Example 2 a small book *** 100 pages, typesetting a lead can only be one digit, please count, row the book's page number *** used how many lead?

solution: classification:

from page 1 to page 9, ***9 pages, each page with 1 lead, *** with 1 × 9 = 9 (a);

from page 10 to page 99, ***90 pages, each page with 2 lead, *** with 2 × 90 = 180 (a);

the 100th page, only 1 page *** with 3 lead So to line up 100 pages of a book . The total number of pages *** using lead characters is:

9+180+3=192(ones).

Example 3 What is the sum of all the numbers used to write all the hundred natural numbers from 1 to 100?

Solution: (see Figure 5-1) First, as required by the question, write out all the one hundred natural numbers from 1 to 100, and then categorize them for calculation:

As shown in Figure 5-1, the wide vertical strips are all single-digit words in the **** there are 10 strips, and the sum of the numbers is:

(1+2+3+4+) 5+6+7+8+9)×10

=45×10

=450.

Each of the narrow vertical strips contains a type of tens digit, *** there are 9 strips, and the sum of the digits is:

1×10 + 2×10 + 3×10 + 4×10 + 5×10 + 6×10 + 7×10

+8×10 +9×10

=(1+2+3+4+5+6+7+8+9)×10

=45×10

=450.

The numerical sum of the other 100 numbers is 1+0+0=1.

So, the numerical sum of these hundred natural numbers is:

450+450+1=

By the way, it is important to draw your attention to the fact that there is often more than one way to solve a math problem, and whoever is able to search for and find a more concise solution is often a sign of greater mathematical ability. For example, there is a more concise solution to this problem, try to see, can you find it?