The charge standard of a city's internal telephone is: 0.2 yuan for the first three minutes (less than 3 minutes is calculated as 3 minutes), and 0.1 yuan for every one minute after that; the charge standard of long-distance telephone is: 0.08 yuan for every 10 seconds (less than 10 seconds is calculated as 10 hours). Xiao Ming made several calls one day continuously, and paid 2.04 yuan in total. How many minutes did Xiao Ming make the most calls

The charge standard of a city's internal telephone is: 0.2 yuan for the first three minutes (less than 3 minutes is calculated as 3 minutes), and 0.1 yuan for every one minute after that; the charge standard of long-distance telephone is: 0.08 yuan for every 10 seconds (less than 10 seconds is calculated as 10 hours). Xiao Ming made several calls one day continuously, and paid 2.04 yuan in total. How many minutes did Xiao Ming make the most calls

Xiao Ming called for 27 minutes and 30 seconds at most
analysis:
Because the charging standard of local calls is lower than that of long distance calls, it is necessary to make as many local calls as possible
Because the charging standard of the local phone call is "0.2 yuan for the first three minutes, and 0.1 yuan for each minute after the call",
If it is impossible for all local calls to have "4 minutes" in the end of the call fee, then the 2.04 yuan call fee includes at least 30 seconds of long-distance call fee: 0.08 × 3 = 0.24 (yuan), then the total local call fee: 2.04-0.24 = 1.8 (yuan)
Considering that the average telephone charge in the city is less than 0.1 yuan per minute in the first three minutes, if 0.1 yuan is added for each call after more than three minutes, it is most cost-effective for each call to take exactly three minutes, then the maximum telephone charge of 1.8 yuan is 18 △ 02 × 3 = 27 (points)

Primary school sixth grade Mathematical Olympiad problem (4) urgent!
1. There are 80 black and white pieces in the box. After Xiao Ming took one seventh of the black pieces and 20 white pieces out of the box, the number of black pieces is twice that of white pieces. How many black pieces are there?
2. Xiao Ming and Xiao Ying have 48 stamps in total. Xiao Ming gives one third of his stamps to his classmates, and Xiao Ying gives three to his classmates. At this time, the number of stamps of Xiao Ying and Xiao Ming is equal. How many stamps did Xiao Ming have?
3. For two bags of rice, the second bag weighs 15 kg more than the first bag. It is known that one third of the weight of the first bag of rice is exactly equal to the second bag of rice. How many kg does each bag weigh?
4. Lao Liu and Xiao Li work together for 12 days. If Lao Liu is allowed to work for 8 days, Xiao Li will do the rest of the work alone, and Xiao Li will have 14 days to finish it. How many days will it take for them to work alone?

1. Suppose there are x sunspots, then there are (80-x) white ones
According to the meaning of the question: X × (1-1 / 7) = 2 × (80-x-20)
The solution: x = 42
80－X=38
A: there are 42 sunspots and 38 white ones
2. Method 1: suppose Xiao Ming has x stamps and Xiao Ying has y stamps
X+Y＝48 (1)
(1-1 / 3) × x = Y-3 (2)
The solution of equation (1) (2) is as follows:
X=27,Y＝21
A: Xiao Ming used to have 27 stamps
Method 2: suppose Xiao Ming has x stamps and Xiao Ying has (48-x) stamps
(1-1 / 3) × x = (48-x) - 3
The solution: x = 27
A: Xiao Ming used to have 27 stamps
3. There is a mistake
4. I won't! Maybe it will take Lao Liu 36 days and Xiao Li 18 days!
Who knows, please tell me, see if I'm right!
When I was in sixth grade, I did these questions every day, but when I was in high school, my thinking was different from that in primary school,

1. A.b.c.d.e is five natural numbers, the sum of AB is equal to 3 / 5 of the sum of the other three numbers, and the sum of De is equal to 5 / 8 of the sum of the other three numbers. If we make the five numbers as small as possible, what should c be?
2. In the reading room, female students account for 4 / 9 of the whole room. Later, two female students came. At this time, female students account for 9 / 19 of the whole room. How many people were there?
Ask for the process to provide

Question 1: the sum of AB is equal to 3 / 5 of the sum of the other three numbers, that is, 3 / 8 of the sum of the five numbers, and the sum of De is equal to 5 / 8 of the sum of the three numbers, that is, 5 / 13 of the sum of the five numbers. In other words, the four numbers of ABDE account for 79 / 104 of the total, then the number of C accounts for 1-79 / 104 of the total = 25 / 104
Question 2: keep the number of boys unchanged
Girls start to account for 4 / 9 of the total, while boys account for 5 / 9, and girls are equivalent to 4 / 5 of boys
Later, girls accounted for 9 / 19 of the total, while boys accounted for 10 / 19
The increase of 2 girls leads to the increase of the score rate from 4 / 5 to 9 / 10, so 2 divided by (9 / 10-4 / 5) = 20 (people), which is the amount of unit 1, that is, the number of boys. Then the number of girls is equal to 20 × 4 / 5 = 16 (people), and the total number is 20 + 16 = 36 (people)