1. First, consider p as a known quantity, solve the binary linear equations 2x-y=8p, 4x+7y=7p to get x=7p/2, y=-p to substitute x and y into 3x-7y=35, and get 3 * 7p/2-7 *(P)= 35.

You can set up an equation to solve it. Suppose that X kilograms of water and Y spoons of washing powder need to be added to the washing machine, and then it can be obtained according to the known equations x+0.02*y+4.94= 15, 0.02*y/ 15=0.4%.

3. First, consider that m and n are known quantities. Since both X and Y satisfy the equation, it holds true for any equation, and so does x-y= 1 and x+y=3. Substitute the calculated x=2, y= 1 into equation 2-m=2n, 2n- 1 =.