1. 450kg of salt water with a salt content of 15%. Now after adding some water, the salt content is 8%. How many kilograms of water?

The amount of salt is constant.

The raw salt is 450* 15%=67. 5 kg

So now the total is: 67. 5/8%=843。 75 kg

That is, adding water: 843. 75-450=393。 75 kg

2. In a math exam, the teacher gave 33 questions, stipulating that 8 points will be deducted for answering one question correctly and 3 points for answering one question incorrectly. Xiaohong answered all the questions and got 0 points. How many questions did Xiaohong answer correctly?

If the total score is: 33*8=264.

Answer a wrong question, less points: 8+3= 1 1.

Now he is less than the total score: 264-0=264 points.

Then the answer is wrong: 264/ 1 1=24 questions.

That is, Xiaohong got it right: 33-24=9 questions.

3. A car goes from A to B. If the speed is increased by 20%, it can take more time than the original time 1 hour; If you drive at the original speed of 120km and then speed up by 25%, you can arrive 40 minutes earlier. How many kilometers is it between A and B?

If the vehicle speed is increased by 20%, the time will be the original:1[1+20%] = 5/6.

Now the time is less than before 1 hour, so the original time is:1[1-5/6] = 6 hours.

Let the original speed of the car be x, then the distance is 6X.

120/X+(6X- 120)/[( 1+25%)X]= 6-40/60

120/X+4 .8-96/X= 16/3

24/X=8/ 15

X=45

That is, the distance between the two places is: 45 * 6 = 270km.