There is 25% sugar water in the container. If 20kg of water is added, the concentration of sugar water becomes 15%. Excuse me, this container originally contained _ _ _ _ _ _ kilograms of sugar.
3. Four kilograms of 30% salt water is made of two kinds of salt water with concentration of 45% and 5%. How many kilograms do these two kinds of brines need?
4. 100g of 5% brine, plus 10% and 15% brine, becomes 200g of 9% brine, and 10% is added with _ _ _ _ _ _ _ _.
Answer analysis
1, when 60 grams of salt water with 30% salt content is evaporated into salt water with 40% salt content, the weight of salt water is _ _ _ _ _ _.
analyse
The difficulty of this problem is to keep the weight of solute salt constant. Beginners can understand it this way: among the 60 students in the class, 30% are boys. When some girls leave, the proportion of boys becomes 40%. How many students are there in the class at this time?
Salt weight (number of boys) = 60× 30% = 18g.
Salt water weight (class size) = 18 ÷ 40% = 45g (corresponding volume rate).
Answer 45
There is 25% sugar water in the container. If 20kg of water is added, the concentration of sugar water becomes 15%. Excuse me, this container originally contained _ _ _ _ _ _ kilograms of sugar.
analyse
The difficulty of this problem lies in finding that the weight of solute sugar is constant, because the logical relationship is complex and can be solved by equation.
Solution: If this container contains X kilograms of sugar, then the original sugar water has (x÷ 25%) = 4 kilograms.
Initial solute mass: x
The later solute mass: (4x+20)× 15%.
x=(4x+20)× 15%
x=7.5
So it originally contained 7.5 kilograms of sugar.
Answer 7.5
3. Four kilograms of 30% salt water is made of two kinds of salt water with concentration of 45% and 5%. How many kilograms do these two kinds of brines need?
analyse
The weight ratio of Party A and Party B is 25%: 15%=5:3.
So the weight of A is 4÷(5+3)×5=2.5 kg, and the weight of B is 4-2.5= 1.5 kg.
Answer 2.5; 1.5
4. 100g of 5% brine, plus 10% and 15% brine, becomes 200g of 9% brine, and 10% is added with _ _ _ _ _ _ _ _.
analyse
This problem is a quadratic mixing problem, which can be solved by concentration crossover, but it will be clearer to solve the logical relationship by equation.
Solution: Assuming that X grams 10% saline is added, the weight of 15% saline is (100-x) grams.
So add 40 g 10% saline.
Answer 40