Show an equation and a solution for the problem. Arnold needs a 25% solution of nitric acid. He has 20 ml of a 30% solution. How many ml of a 15% solution should he add to obtain the required 25% solution?
A. 20 ml
B. 15 ml
C. 25 ml
D. 10 ml

Question

Show an equation and a solution for the problem. Arnold needs a 25% solution of nitric acid. He has 20 ml of a 30% solution. How many ml of a 15% solution should he add to obtain the required 25% solution?
	A.	20 ml
	B.	15 ml
	C.	25 ml
	D.	10 ml

anonymous · Answer

at the moment he has 30% of 20 which is $.3\times 20=6$ ml of whatever is in the solution
if he adds say $x$ ml of 15% solution it will contribute $.15x$ so the total of whatever is in the solution will be $6+.15x$ 
on the other hand he will have $20+x$ ml of solution all together which he wants to be $25\%$ whatever
so set 
$$6+.15x=.25(20+x)$$ and solve for $x$

anonymous · Answer

probably easiest to get rid of the decimals by multiplying both sides by 100 and solve 
$$600+15x=25(20+x)$$ etc