Question 395040
Let {{{a}}} = amount of 90% mixture to use
Let {{{b}}} = amount of water to add
In words:
(total juice)/(total mixture) = 50%
{{{.9a/(a + b) = .5}}}
There are 2 unknowns and only 1 equation, so it's
not solvable unless I know how much 50% mixture 
I end up with, then I can write the 2nd equation:
{{{a + b}}} = amount of 50% mixture
However, I can show that:
{{{.9a/(a + b) = .5}}}
{{{.9a = .5a + .5b}}}
{{{.4a = .5b}}}
{{{4a = 5b}}}
{{{a/b = 5/4}}}
So, the ratio of added water to 90% mixture is  5/4
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I don't have a clue about adding sugar