book Maths Puzzle Numerical Ability

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Solve for a s=1/2(a+b+c)

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book Other Question

i need 1000 mils of 10% acid solution. i have 5% and 25% available. how much of each do i need to make the 1000 mils?

x=5% mils
y=25% mils
5x+25y=1000 this is as far as i c I need help in doing this.
For which system(s)is (2,3)the solution?
I know I have to plug in 2 for x and 3 for y
A)y = 2x-1 and y = 1/2x +2
B)y = 1/4x + 5/2 and y = -3x + 3
C)y = x + 1 and y = -x + 5
D)y = 3x - 3 and y = -2x + 7