The correct answer is (D)
To calculate this answer, first note down the information presented to keep track of the various parts. k for potassium, p for phosphorus and n for nitrogen.
Blend 1 is 40% nitrogen (n) + 60% potassium (k)
Blend 2 is 80% phosphorus (p) + 20% nitrogen (n)
The final mixture is Blend 1 (x) + Blend 2 (y) = 24 kgs
Nitrogen -> we know the percentages for the intial two blends and together can determine the AMOUNT based on the final 24 kg weight. We also know that would be 35%, so we can determine the phosphorus percentage and amounts from that.
40%n + 20%n = 35%n x 24kgs -> 0.4n + 0.2n = 0.35n x 24 = 8.4 kgs of nitrogen in the final blend
Final Blend -> we need to determine the breakdown of Blend 1 and Blend 2 that make up the final Blend so that we can determine the final amount of phosphorus. We can work backwards from the nitrogen values to do so using substitution:
x + y = 24
x = 24 - y
Substitute into the nitrogen equation
0.4x + 0.2(24-x) = 8.4 kgs
0.4x + 4.8 - 0.2x = 8.4
0.2x + 4.8 = 8.4
0.2x = 3.6
x = 18 kg (Blend 1)
if the final Blend is 24 kgs, Blend 2 is 24 kgs - 18 kgs = 6 kgs
Phosphorus is only in Blend 2, so we use the 80% of the 6 kgs to determine that Blend 2 has 4.8 kg of Phosphorus.
Therefore the final blend will have the same 4.8 kg of Phosphorus, answer choice D.