One of the questions in today s test The answer to this question?

Of course it can't be calculated!

To put it in terms of elimination: there is no explanation of weight in the question, and the answer naturally cannot be A or B; Doubling the mass fraction and evaporating half of the solvent is obviously the most mentally retarded answer; (The previous mass fraction was 1 10, and it was doubled to 1 5, assuming that the solution was 10, the solute was 1, and the solvent was 9, and half of the solvent was evaporated, that is, 1 (.)

Pick D! Just choose D, that's what I think anyway

Option B assumes that the original solvent mass is 10g and the solution mass is 100g.

After the addition of solvent, his mass fraction is calculated as:

10g+ is double the mass fraction. is 20%.

Pick D! You're right. If the solution is saturated after the mass fraction is doubled, generally speaking, the added solute mass is 1 8 of the mass of the original solution, and the evaporated solvent mass is 1 2 of the solution mass

Pick D! First of all, if the quality of the solution is not known, AB cannot be discussed.

For c, let the solute mass be x, then the solvent mass is 9x, and after evaporation, half of the mass fraction is x (x+, not 20%)

Do you still need to think about it? Of course it's C!

According to the title, there are x students with the following preparations.

5x+3=7x-5

Solving this equation has 2x=8, i.e., x=4. There are 4 students and 23 dusty pencils. Brother Zen ruined.

4 students, 23 pens.

The equation only needs to be set to x

The equation is: 5x+3=7x-5

Set up the production of X pieces of A products and 50-x pieces of B products. then 9x+4(50-x)<=360, 3x+10(50-x)<=290

Xie Fu learned: x "Hu Hall Shooting=32,x>=30 Three kinds of square envy cases: A30, B20; a31,b19;a32,b18.

Haha....Test me....After the waiter took two dollars, the three of them spent a total of 28 yuan, so in fact, each person did not spend 9 yuan at all, but 28 3!

3 times 9 = 27

27+3 equals 30.

Not a dollar less ......

It should be an approximate number, and it is easier to understand that the average number is larger.

A: Mileage reduced by 10 percent.

With 565-350 = 215km

Divide 215 by 565

Count the rest yourself.

Solution: 3 3=9,9 3=27,27 3=81,81 3=243,243 3=729....Continuing like this, we find a pattern:

The single digit changes periodically: 9, 7, 1, 3, 9, 7, 1, 3, 9 Thus, after the 100th 3 multiplied by the single digit is 1, and then subtracted by 5, then the final result is 6

Set up x people in the middle of the Guohuai Niantuan, and the high can list the equation: lead orange.

40-x+7=45

Solution: x=2

So 2 people got off halfway.

After reading the problem and missing the eye, set the equation.

Pick-up: Set x people to get off halfway.

40-x+7=45

Moved 47 to 45 and became -47.

x=-2x=2

Then the one who got off halfway was the Pei type 2 people.

Let the midway train be x, and according to the question, 40-x+7=45 x=2