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Subjects
Class 9MathematicsIntroduction to PolynomialsLinear growth and decayMediumLevel 37
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Water left in a tank is (W(t)=180-12t) litres. How much water will remain at (t=9)?

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Answer and explanation

Correct answer: 72 litres

Given \(W(t)=180-12t\). Substituting \(t=9\), \(W(9)=180-12(9)=180-108=72\) litres. Therefore, 72 litres is correct. The nearby distractor 84 litres does not result from substituting 9 correctly in the given expression. Exam tip: in a linear decay expression, substitute the time value first, multiply, and then subtract the decrease from the initial amount.

Related tags

Linear DecayLinear PolynomialSubstitutionAlgebraic ExpressionsWord Problems

Frequently asked questions

What is the correct answer to this question?

72 litres

Why is this the correct answer?

Given \(W(t)=180-12t\). Substituting \(t=9\), \(W(9)=180-12(9)=180-108=72\) litres. Therefore, 72 litres is correct. The nearby distractor 84 litres does not result from substituting 9 correctly in the given expression. Exam tip: in a linear decay expression, substitute the time value first, multiply, and then subtract the decrease from the initial amount.

Which subject and chapter does this question cover?

This is a Class 9 Mathematics question. Chapter: Introduction to Polynomials. Topic: Linear growth and decay.

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10 more MCQs
  1. 1If the height of a plant is (h(t)=20+3t) cm, what type of change does it show?
  2. 2If the amount of water is (W(t)=50-4t) litres, what type of change is it?
  3. 3In the linear rule (C(d)=15+2d), what is the initial value?
  4. 4In the linear rule (P(t)=80-5t), what is the rate of change per unit?
  5. 5If (S(t)=10+6t), what will be the value of (S(t)) at (t=4)?
  6. 6If (M(t)=90-7t), what is the value of (M(t)) at (t=5)?
  7. 7Books in a library increase according to (B(n)=100+8n). What does the coefficient of (n) show?
  8. 8If water left in a tank is (L(t)=60-3t) litres, how much water was there at (t=0)?
  9. 9Which rule represents linear growth?
  10. 10Which rule represents linear decay?
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