let a milliliters = amount of 20% solution used
let b milliliters = amount of 0.05% solution used
Then a + b = 150 (total amount used = 150) (*)
and 0.2 a + 0.005 b = 150 x 0.07 ( concentrations before and after mixing is same)
or 20a + 0.5b = 1050 (multiplying by 100 )
or 40a + b = 2100 (multiplying by 2 )
but a + b = 150 from equation (*)
So 39 a = 1950 (subtracting the two equations)
a = 1950/39 = 50
Hence 50milliliters of 20% solution should be mixed with 100 milliliters of 0.005 % solution to get 150milliliters of 7 % solution
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