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The solution by @josgarithmetic is T O T A L L Y W R O N G.
Below find a correct solution.
Let x be the volume of water to be added, in gallons.
Then the volume of the 8% plant fertilizer solution to use is (0.5-x) gallons.
The 8% plant fertilizer solution contains 0.08*(0.5-x) gallons of the pure fertilizer.
Divided by the total volume of 0.5 gallons, it should give the 2% concentration
= 0.02.
Simplify by multiplying both sides by 0.5
0.08*(0.5-x) = 0.5*0.02 = 0.01.
It gives you
0.5 - x = = , and then
x = 0.5 - = gallons of water.
Answer. gallons of water should be added, which is exactly 3 pints of water.
Solved.
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Compare it with the answer by @josgaritmetic, who gives 1 pint of water.
@josgarithmetic is VERY well known (or better to say, very badly known, unfortunately) in this forum,
for his inability to solve problems and make calculations correctly.
So be very careful when you obtain "solutions" from him.
In half of all cases they are wrong.
Do not trust him and always wait for reaction of other tutors.
In the normal school environment he would be fired in the second day.