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In solving mixture problems, you have to determine how much "pure" stuff you need.
For adults and children, you need 3 oz of fluid.
The child's the solution must be 50% glucose.
So, the child's 3 oz will need to have 1.5 oz of "pure" glucose and 1.5 oz of other stuff (likely water, but it doesn't matter).
You have on hand 80% solution.
So for each oz you have .8 oz of glucose.
To get 1.5 oz of pure glucose, you need:
.8x = 1.5
8x = 15
x = 15/8
x = 1 7/8 oz of the 80% solution
You need 3 oz in total, so add 1 1/8 oz of water to 1 7/8 oz of of the 80% glucose solution to produce 3 oz of 50% solution.