Formula:

The amount of grass eaten by each cow per day is assumed to be 1.

How much grass did A eat in the first b days?

How much grass did M eat in the first n days?

Subtract the big one from the small one and divide it by the difference of the corresponding days.

The result is the growth rate of grass.

The original amount of grass is correspondingly reversed.

The formula is the amount of grass eaten by A on the first day minus the amount of grass eaten on the second day multiplied by the growth rate of grass.

Cattle with unknown grazing amount are divided into two parts:

A small number eat new grass first, and the number is the proportion of grass;

Divide some grass by the number of remaining cows to get the required number of days.

The grass grows thick and fast all over the pasture. 27 cows can eat grass for 6 days; 23 cows can eat the grass in 9 days. Q 2 1 How many days will it take to finish the grass?

Assume that the daily grazing amount of each cow is 1, the grazing amount of 27 cows for 6 days is 27×6 = 162, and the grazing amount of 23 cows for 9 days is 23× 9 = 207. .

Subtract the small from the large, 207-162 = 45; The difference between two corresponding days is 9-6=3 (days), and the result is the growth rate of grass. So the growth rate of grass is 45/3= 15 (cattle/day); The original amount of grass is correspondingly reversed.

The formula is the amount of grass eaten by A on the first day minus the amount of grass eaten on the second day multiplied by the growth rate of grass. Therefore, the amount of grass =27X6-6X 15=72 (cattle/day).

Cattle with unknown grazing amount are divided into two parts: a small part eats new grass first, and the quantity is the proportion of grass;

That is to say, the required 2 1 cow is divided into two parts, one part is 15 cows eating new grass; The remaining 2 1- 15=6 eats raw grass, so the number of days needed is: the number of raw grass/the distribution of remaining cattle =72/6= 12 (days).

12, age problem

Formula:

The precession will not change, when adding and subtracting.

With the change of age, the multiple is also changing.

Grasp these three points and everything will be easy.

Example 1: Xiaojun is 8 years old and his father is 34 years old. A few years later, his father was three times older than Xiaojun.

The precession will not change, this year's age is almost 34-8=26, and it will not change in a few years.

Knowing the difference and multiple, it becomes the problem of difference ratio. 26/(3- 1)= 13. In a few years, dad's age will be 13X3=39, and Xiaojun's age will be 13X 1= 13, so it should be five years later.

Example 2: Sister 13 years old, brother 9 years old. How old should they be when the sum of their ages is 40?

The precession will not change, and the age difference 13-9=4 this year will not change in a few years.

After several years, the age sum is 40, and the age difference is 4, which turns into a sum-difference problem. Then a few years later, the elder sister's age is (40+4)/2=22 and the younger brother's age is (40-4)/2= 18, so the answer is 9 years later.

13, remainder problem

Formula:

There are (N- 1) remainders, the smallest is 1 and the largest is (N- 1).

When the cycle changes, don't look at the quotient, just look at the surplus.

For example, if the clock now shows 18 o'clock, what time will it be after the minute hand turns 1990? One revolution of the minute hand is 1 hour, and 24 revolutions is 1 circle of the hour hand, that is, the hour hand returns to its original position.

The remainder of 1980/24 is 22, so it is equivalent to the minute hand turning 22 times forward, which is equivalent to the hour hand moving 22 hours forward, which is equivalent to 24-22=2 hours backward, which is equivalent to the hour hand pulling back 2 hours. The instantaneous needle is equivalent to 18-2= 16 (point).