Elementary Mathematics, Seeking Analysis, Elementary Mathematics, Solving

3. It takes 9 hours for A to do alone, 12 hours for B to do alone, 1 9 per hour for A to complete the project, and 1 12 for B to complete the project per hour

In 2 hours, A and B take turns to do it, A does 1 hour, and B also does 1 hour, and a total of 1 9 + 1 12 = 7 36 of the project is completed

That is to say, in 2 hours, A and B completed a total of 7 36 of the project

Therefore, the number of times A and B need to take turns to complete the whole project = 1 (7 36) = 36 7 = 5 and 1/7 times.

A and B take turns to do it 5 times, which is 2*5=10 hours, and there is still 1-(7 36)*5=1-35 36=1 36 of the remaining project

At this time, it is A's turn to do it, and it will take (1 36) (1 9) = 1 4 = hours.

So. It takes 10+ hours to complete the entire project.

4. The total number is certain, and the time is inversely proportional to the ergonomics, so.

The ergonomic ratio of A and B is 5:3 (that is, the ratio of the total number of completions completed by A and B at the same time).

5 (5+3) = 5 8 A completes 5 8 of the total

66 (5 8-3 7) = 336 units "1" is the total number of parts in the batch.

Rectangular pool volume:

120, cubic meters), 2 dm = m, the radius of the outlet pipe is: m), the volume of water per minute is:

cubic meters), minutes).

A: It takes 50 minutes to drain the water in the pool.

s1=1/(1×3)=1/3

s2=1/3+1/15=2/5

s3=2 5+1 35=15 35=3 7So:sn=n (2n+1).

Mathematical induction is not an elementary school curriculum, it is a high school curriculum.

The 15th row has: 20 + 2 * (15-1) = 48 (pcs) The 1st row plus the 15th row: 20 + 48 = 68

。Add 68 from beginning to end

The middle number of rows 1 to 15 is 68 2 = 34 total seats 34 * 15 = 510

Excluding the first row, there are 15-1=14 rows of seats.

14 rows more 20+2+2+......2 = 210 seats.

Number of seats per row of 20 people: 20 15 = 300

Total number of seats: 300 + 210 = 510

Suppose a cow eats grass for one day, and 5 cows eat for 40 days: 1 5 40 = 200 (unit one).

6 cows ate in 30 days: 1 6 30 = 180 (unit one) difference: 200-180 = 20 (unit one) This is 40-30 = 10 (days) of grass.

The grass grows in one day: 20 (40-30) = 2 (unit one) The original grass in the grassland: 200-2 40 = 120 (unit one) 30 days long: 2 30 = 60 (unit one).

Plus the original: 120 + 60 = 180 (unit one) 4 cows ate for 30 days, eaten: 1 4 30 = 120 (unit one) 4 cows ate for 30 days left:

180-120 = 60 (unit one) plus 2 cows: one cow eats 1 unit one a day, and 2 cows eat: 1 2 = 2 (unit one), which is exactly the grass growth rate, so the remaining 60 units of one are given to four cows:

60 (1 4) = 15 (days).

Let the grass that grows in the pasture every day be x

Then 40 5 = original grass + x 40

30 6 = original grass + x 30

x×10=200-180

x = 2 turns out to have grass = 120

4 cows were left after 30 days of eating.

Set the rest of the grass to eat clean.

6y=60+2×y

y=15 days.

Did you learn equations? It's easy to understand when you learn it.

Let B's velocity be x

Then there is: (6- x)*200=runway perimeter.

6+x)*40 = runway perimeter.

That is: (6-x)*200=(6+x)*40

Solution: x=4

So: runway circumference = 400m

Diameter = 400

To put it bluntly, it is the problem of pursuit and encounter in the Olympiad of primary school.

Assuming the runway length x meters.

A overtakes B in 200 seconds.

In 200 seconds, A walked 200*6=1200 meters.

Then B's velocity is (1200-x) 200

Meet in 40 seconds.

40*(6+(1200-x)/200)=x40(12-x/200)=x

480-x/5=x

480=6x/5

x = 400 meters.

Then the diameter d = 400 3 meters.

Solution: Let the speed of B be x meters per second, and the runway is y meters long.

3 minutes 20 seconds = 200 seconds.

From this we get two equations: 6 200-200x=y6 40+40x=y

There are two equations above, because the right side of the equation is y, so 6 200-200x = 6 40 + 40x

Solve the equation and get x=4

Substituting x=4 into either of the above two equations gives y=400, so the runway diameter is divided by 400.

(v A + v B) times 40 = (v A - v B) times 2002 v A = 3v B.

v B = 4 sec.

2r=400/π

I don't understand why the first two 400 3s are agreed, do the elementary school teachers say that they all use 3 during elementary school, and the third is easier to understand, one is more than one lap, and the other is a lap together.

Question 4:

Question 5: The upper left corner of the diamond is blackened.