Question: 200 grams of 60% alcohol solution and 300 grams of 30% alcohol solution are mixed, and the concentration of the obtained alcohol solution is ().
Analysis:
Solution mass = solute mass+solvent mass
Solute mass = solution mass × concentration
Concentration = solute mass ÷ solution mass
Solution mass = solute mass/concentration
To require the concentration of mixed solution, we must find out the total mass of mixed solution and the mass of pure alcohol.
The total mass of the mixed solution is the sum of the mass of the first two solutions:
200+300=500 grams.
The content of pure alcohol after mixing is equal to the sum of pure alcohol in the two solutions before mixing:
200× 60%+300× 30% =120+90 = 210 (g)
The concentration of the mixed alcohol solution is:
2 10÷500=42%
A: The concentration of mixed alcohol solution is 42%.
Jin Dian: When two solutions with different concentrations are mixed, the total amount of solution and solute remains unchanged.
Questions A, B and C plant trees in plots A and B, with 900 trees in plot A and 0/250 trees in plot B. It is known that Party A, Party B and Party C can plant 24, 30 and 32 trees each day. Party A plants trees in Site A, Party C plants trees in Site B, Party B plants trees in Site A first, and then goes to Site B ... The two plots start and end at the same time. On what day should B be transferred from A to B?
The total number of plants is 900+ 1250 = 2 150, and 24+30+32 = 86 plants can be planted every day.
The planting days are 2 150 ÷ 86 = 25 days.
24× 25 = 600 trees will be completed in 25 days.
Then B will finish 900-600=300 trees before helping C.
That is, 300 ÷ 30 = 10 days later.
That is, on 1 1 day, it was transferred from a place to b place.
A project, contracted by both parties, can be completed in 2.4 days and needs to be paid 1800 yuan; Contracted by Team B and Team C, it can be completed in 3+3/4 days, and it needs to be paid 1500 yuan; 2+6/7 days, cost 1600 yuan. On the premise of ensuring the completion within one week, which team should be selected for the lowest cost?
Analysis of one-day cooperation between Party A and Party B: 1 ÷ 2.4 = 5/ 12, 1800 ÷ 2.4 = 750 yuan.
The cooperation between ethylene, propylene and Fang Yitian is 1 ÷ (3+3/4) = 4/ 15, and the payment is 1500× 4/ 15 = 400 yuan.
The cooperation between Party A, Party C and Fang Yitian is 1÷ (2+6/7) = 7/20, and the payment is 1600× 7/20 = 560 yuan.
Three people cooperate in one day (5/12+4/15+7/20) ÷ 2 = 31/60,
Three people cooperate to pay (750+400+560) ÷ 2 = 855 yuan a day.
Party A alone completes 31/60-4/15 =1/4 every day, and pays 855-400 = 455 yuan.
Party B alone completes 3 1/60-7/20 = 1/6 every day, and pays 855-560 = 295 yuan.
Party C alone completes 31/60-5/12 =110 every day and pays 855-750 = 105 yuan.
So by contrast,
Choose b with 1 ÷ 1/6 = 6 days, only 295× 6 = 1770 yuan.
There are three grasslands with areas of 15 and 10 respectively. Eat 28 cows in 80 days?