Answer is 1.2 pints fertilizer
Step by step
First I’m going to change 2 quarts to 4 pints because we are going for a smaller portion of water to fertilizer
Next you find your rate
50 gal water to 4 pints fertilizer = 12.5 rate
We want to solve for x, 15 gallons water to x fertilizer
50/2 to 15/x
Substitute your rate for x and divide
15/12.5 = 1.2 pints