Example

Solving a Diet Calorie and Budget Problem Using a System of Inequalities

Problem: Philip's doctor tells him he should add at least 1{,}000 more calories per day to his usual diet. Philip wants to buy protein bars (costing $1.80 and having 140140 calories each) and juice (costing $1.25 per bottle and having 125125 calories each). He does not want to spend more than $12.

ⓐ Set up the system. Let pp = the number of protein bars and jj = the number of bottles of juice. Translating the two constraints:

  • "At least 1{,}000 calories" → 140p+125j≥1,000140p + 125j \geq 1{,}000
  • "No more than $12" → 1.80p+1.25j≤121.80p + 1.25j \leq 12

Because quantities cannot be negative, we also have p≥0p \geq 0 and j≥0j \geq 0. The system is: {140p+125j≥1,0001.80p+1.25j≤12p≥0j≥0\left\{\begin{array}{l} 140p + 125j \geq 1{,}000 \\ 1.80p + 1.25j \leq 12 \\ p \geq 0 \\ j \geq 0 \end{array}\right.

ⓑ Graph the system. Graph 140p+125j=1,000140p + 125j = 1{,}000 as a solid boundary line. Testing (0, 0): 0≥1,0000 \geq 1{,}000 is false, so shade the side away from the origin. Graph 1.80p+1.25j=121.80p + 1.25j = 12 as a solid boundary line. Testing (0, 0): 0≤120 \leq 12 is true, so shade the side containing the origin. The solution is the doubly-shaded region in the first quadrant.

ⓒ To determine if 33 protein bars and 55 bottles of juice satisfy the needs, we test the point (3, 5) in the inequalities:

  • Calories: 140(3)+125(5)=420+625=1,045≥1,000140(3) + 125(5) = 420 + 625 = 1{,}045 \geq 1{,}000 (True)
  • Budget: 1.80(3)+1.25(5)=5.40+6.25=11.65≤121.80(3) + 1.25(5) = 5.40 + 6.25 = 11.65 \leq 12 (True) Since both constraints are met, he can buy 33 protein bars and 55 bottles of juice.

ⓓ To determine if 55 protein bars and 33 bottles of juice satisfy the needs, we test the point (5, 3):

  • Calories: 140(5)+125(3)=700+375=1,075≥1,000140(5) + 125(3) = 700 + 375 = 1{,}075 \geq 1{,}000 (True)
  • Budget: 1.80(5)+1.25(3)=9.00+3.75=12.75≤121.80(5) + 1.25(3) = 9.00 + 3.75 = 12.75 \leq 12 (False) Since the budget constraint is not met, he cannot buy 55 protein bars and 33 bottles of juice.

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Updated 2026-06-05

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