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put this solution on YOUR website!Let T be the volume of 30% solution, F be the 50% solution.
Total volume
1.

Total concentration
2.

2.

Multiply eq.1 by -30 and add to eq. 2 and solve for F,

From 1,
1.

.
.
.
20mL of 30% solution, 30 mL of 50% solution.