Step 1
It is known that a value of "x" % (m/m) means:
"x" grams in 100 grams of solution
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Step 2
Information provided:
3.9 % (m/m) of NaCl solution
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Step 3
Procedure:
According to step 1, it is known that:
3.9 % (m/m) => 3.9 g NaCl in 100 g
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Now,
3.9 g NaCl ------------ 100 g of solution
7.20 NaCl ------------- X
X = 7.20 NaCl x 100 g of solution/3.9 g NaCl
X = 184.61 g = 180 g approx.
Answer: 180 g of solution