Maximizing Linear Combination Function

XAT 2020 · Quantitative Ability & Data Interpretation · Medium · Quantitative Aptitude

Consider the four variables A, B, C and D and a function Z of these variables, Z=15A2−3B4+C+0.5DZ = 15A^2 - 3B^4 + C + 0.5D. It is given that A, B, C and D must be non-negative integers and that all of the following relationships must hold: i) 2A+B≤22A + B \le 2 ii) 4A+2B+C≤124A + 2B + C \le 12 iii) 3A+4B+D≤153A + 4B + D \le 15

If Z needs to be maximised, then what value must D take?

  1. A.

    15

  2. B.

    12

  3. C.

    0

  4. D.

    10

  5. E.

    5

Answer

B

Explanation

To maximize Z=15A2−3B4+C+0.5DZ = 15A^2 - 3B^4 + C + 0.5D, we want B=0B = 0 (since −3B4-3B^4 severely penalizes ZZ). If B=0B = 0: From (i) 2A≤2  ⟹  A=0 or 12A \le 2 \implies A = 0 \text{ or } 1. Case 1: A=1,B=0A = 1, B = 0. From (ii) 4(1)+C≤12  ⟹  C≤8  ⟹  Cmax⁡=84(1) + C \le 12 \implies C \le 8 \implies C_{\max} = 8. From (iii) 3(1)+D≤15  ⟹  D≤12  ⟹  Dmax⁡=123(1) + D \le 15 \implies D \le 12 \implies D_{\max} = 12. Z=15(1)2−0+8+0.5(12)=15+8+6=29Z = 15(1)^2 - 0 + 8 + 0.5(12) = 15 + 8 + 6 = 29.

Case 2: A=0,B=0A = 0, B = 0. C≤12,D≤15  ⟹  Z=0+12+0.5(15)=19.5C \le 12, D \le 15 \implies Z = 0 + 12 + 0.5(15) = 19.5.

Thus, ZZ is maximized when A=1,B=0,C=8,D=12A = 1, B = 0, C = 8, D = 12.

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