What does (1-1 / 2) x (1-1 / 3) x... X (1-1 / 19) x (1-1 / 20) equal fast

What does (1-1 / 2) x (1-1 / 3) x... X (1-1 / 19) x (1-1 / 20) equal fast

(1-1/2)X(1-1/3)X...X(1-1/19)X(1-1/20)
=1/2X2/3X3/4.19/20
=1/20
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Find the positive integer solution of equation 1 / x + 1 / x + 1 + 1 / x + 2 + 1 / x + 3 = 19 / 20
I can't understand the following process
If x is a positive integer, first look at the range, the left side is less than 4 / X and greater than 4 / (x + 3). First solve the equation 4 / x = 19 / 20, and get x = 4.2
If 4 / (x + 3) = 19 / 20, x = 1.2
Then the range of positive integer solution is 1.2 ≤ x ≤ 4.2
The possible values of X are 2,3,4, each of which is taken into account, and the final solution is x = 3

Because the greater the denominator, the smaller the fraction, so x, x + 1, x + 2, x + 3 are greater than or equal to x, less than or equal to x + 3, so the fraction 1 / x, 1 / (x + 1), 1 / (x + 2), 1 / (x + 3) are less than or equal to 1 / x, greater than or equal to 1 / (x + 3) add them up to 1 / (x + 3) + 1 / (x + 3) + 1 / (x + 3) ≤ 1 / x + 1 / (x + 1) + 1 / (x + 2) + 1 / (x +...)

How to solve the equation that 30x equals 20 times minus x

=10x / 7 - {[(17-20x) / 10] / 3 / 10} = 10x / 7 - [(17-20x) / 3] = (30x-119 140x) / 21 = (170x-119) / 21 multiplied by 0.21 at the same time, the equation is changed to 0.21

25 percent of a number is equal to the sum of one fourth and one sixth

(1/4+1/6)/25%=(3+2)/12/0.25=5/(12*0.25)=5/3

60% of a number plus 5 equals 1 / 2 of 50?

（50×1/2-5）÷60%=100/3

A number minus 32 is equal to 25% of 96?

96*25%+32
=24+32
=56

If you subtract 32 from a number, it's 25 percent of 80

80 times 25 = 20 + 32 = 52

The sum of a number and 4 out of 5 is equal to 25 percent of 32

Let this number be X
x+4/5=32*25%
X = 7 and 1 / 5

Use the equation to describe the equality between (1) the number x is less than two-thirds of it. The difference between two times of the number x and - 2 is equal to 10
Use the equation to describe the equality between the following quantities
(1) The number x is one less than two-thirds of it
(2) The difference between two times the number x and - 2 is 10
(3) The sum of 20% of X and four fifths of Y is 44
(4) The price of a drug after 25% price increase is 50 yuan. If the price of the drug before the price increase is x yuan, the equation can be obtained

(1) The number x is one less than two-thirds of it
2/3x-x=1
(2) The difference between two times the number x and - 2 is 10
2x-(-2)=10
(3) The sum of 20% of X and four fifths of Y is 44
20%x+4/5y=44
(4) The price of a drug after 25% price increase is 50 yuan. If the price of the drug before the price increase is x yuan, the equation can be obtained
(1+25%)x=50

4 / 7 of the number a is equal to the number B. the number a is 28. What is the number B? (list the quantitative relationship, do not use the equation)

The quantitative relationship of this question is as follows
Take number a as unit 28, and number B is 28 (4 / 7) = 16