Question 257791
Sally wants to know how much water to add to a 40 ml of 70% acid to make a concentration of 50% acid. 
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Equation:
acid + acid = acid
0.70*40 + 0*x = 0.50(x+40)
Multiply thru by 100 to get:
70*40 + 0 = 40x + 50*40
40x = 20*40
x = 20 ml (amount of water to add)
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Cheers,
Stan H.