The sixth grade students signed up to participate in the mathematics interest group. The number of students who participated was 13 of the total number of sixth grade students. Later, 20 students participated. At this time, the ratio of the number of students who participated and those who did not participated was 3:4______ People

20 (33 + 4 − 13) = 20 (221) = 210 (people) a: there are 210 people in Grade 6

The sixth grade students signed up to participate in the mathematics interest group. The number of students who participated was 13 of the total number of sixth grade students. Later, 20 students participated. At this time, the ratio of the number of students who participated and those who did not participated was 3:4______ People

20 (33 + 4 − 13) = 20 (221) = 210 (people) a: there are 210 people in Grade 6

The sixth grade students signed up to participate in the mathematics interest group. The number of students who participated was | / 3 of the total number of sixth grade students. Later, 20 students participated. At this time, the ratio of the number of students who participated and those who did not was 3:4?

20 (3 / 7-1 / 3) = 210

The sixth grade students signed up to participate in the mathematics interest group, one third of the total number of sixth grade students, and then 20 students joined the group
The ratio of non participants is 3:4
(2) Cut a round piece of paper and put it together into an approximate rectangle. The circumference of the rectangle is 16.56 cm. Find out the area of the original round piece of paper in square meters

According to "there were 20 participants later, and the ratio of the number of students who participated and those who did not participated was 3:4", it can be seen that there were 3 students who participated, 4 students who did not participate, and 3 / (3 + 4) = 3 / 7 of the total number of students in Grade 6
So the corresponding score of 20 people is: 3 / 7-1 / 3
To find the total number of sixth grade students is to find the unit "1" by Division: 20 / (3 / 7-1 / 3) = 210

Can you find 8 numbers from 1 to 100 integers and make their reciprocal sum equal to 1?

∵1=1-12+12-13+13-14+14+… +18=（1-12）+（12-13）+（13-14）+… +(17-18) + 18 = 12 + 16 + 112 + 120 + 130 + 142 + 156 + 18, the 10 numbers can be: 2, 6, 8, 10, 12, 20, 30, 42, 56

How much is 12 out of 17 times 34 out of 9 minus 4 out of 9

12/17x34/9-4/9=24/9-4/9=20/9

How much is 15 out of 34 times 17 out of 25?

Three tenths

28, 34, 8, 25, 16, 17, 14, 48, 9 add up to 100

28+8+48+16=100

What's 25% of the difference between 57 and 17?

16. 17, 18, 19 (each number is used once), make up four mixed formulas of addition and subtraction

（16+18）-19+17
19-（16+17）+18
17-（18+19-16）
（19-16）+17-18