Question 635359
20 gallons contains 15%

so it contains 3 grams salt.   ((15/100) *20)

We need 12 % salt

so we have to add

(3*100)/12

=25 gallons

you have to add 25-20 = 5 gallons of water to make it 12% solution.

Alternative

-------------- 	percent	----------------	Amount									
Salt solution	15	----------------			20	gallons						
Water	0	----------------			x	gallons						
Mixture	12	----------------	20	+	x	gallons						
												
15	*	20	+	0	x	=	12	(	20	+	x	)
300	+	0	x	=	240	+	12	x				
0	x		-12	x	=	-300	+	240				
-12	x	=	-60									
/	-12											
x	=	5		gallons	Water							
												
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