2. Because only sugar is added and no water is added, the total amount of water remains unchanged. So 40 * 20% = 8g, which is the content of sugar, and 40-8 = 32g, which is the amount of water. So suppose that the added sugar is x (8+x)/(32+x)=40%, and X = 8g is obtained.
3. The price of mixed sugar is the total price/quality, so suppose that X kilograms of sugar B is needed, and12 * 4+5 * x = 4.2 * (12+x) gets x=3 kilograms.
4. Because the last remaining 80 yuan of B is 4/5 of his, B is 100 yuan. Let A be x, then x-(3/7)*( 100+x)=80, and the solution is x=2 15 yuan.
There is no solution to this problem, so the condition should be missing.
6. Suppose it takes X hours for the upstream and 2-x hours for the downstream, and the total distance is 7x+8(2-x), but there is no other data, so the length of the total distance cannot be known, so this problem has no solution.