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Mathematics

Case Study II

A bus travels at a certain average speed for a distance of 75 km and then it travels a distance of 90 km at an average speed of 10 km/hr more than the original speed. If it takes 3 hours to complete the total journey, then based on this information, answer the following questions:

1. If the original speed of the bus be x km/hr, then time taken by the bus to travel the next given distance is:

  1. 75x\dfrac{75}{x} hours

  2. 90x\dfrac{90}{x} hours

  3. 90x+10\dfrac{90}{x + 10} hours

  4. 90x10\dfrac{90}{x - 10} hours

2. The quadratic equation for the given information, if the original speed of the bus be x km/hr, is:

  1. x2 + 45x − 250 = 0
  2. x2 − 45x − 250 = 0
  3. x2 − 75x − 450 = 0
  4. x2 − 45x + 250 = 0

3. The original speed of the bus is:

  1. 50 km/hr
  2. 40 km/hr
  3. 75 km/hr
  4. 60 km/hr

4. The speed of the bus during which it travels the distance of 90 km is:

  1. 70 km/hr
  2. 50 km/hr
  3. 60 km/hr
  4. 85 km/hr

5. The time taken by the bus to travel a distance of 510 km with the new speed is:

  1. 8 hours

  2. 8 12\dfrac{1}{2} hours

  3. 10 15\dfrac{1}{5} hours

  4. 12 34\dfrac{3}{4} hours

Quadratic Equations

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Answer

1. Given,

Original speed of the bus = x km/hr

Next given distance = 90

Speed for next distance = (x + 10) km/hr

Time = DistanceSpeed\dfrac{\text{Distance}}{\text{Speed}}

Time taken for next distance = 90(x+10)\dfrac{90}{(x + 10)}.

Hence, option (3) is the correct option.

2. Time taken by bus to travel 75 km = 75x\dfrac{75}{x}

Time taken by bus to travel 90 km = 90x+10\dfrac{90}{x + 10}

Given,

Total time taken to complete the journey = 3 hours

75x+90x+10=375(x+10)+90xx(x+10)=375x+750+90xx2+10x=3750+165x=3(x2+10x)0=3x2+30x165x7503x2135x750=03(x245x250)=0x245x250=0.\Rightarrow \dfrac{75}{x} + \dfrac{90}{x + 10} = 3 \\[1em] \Rightarrow \dfrac{75(x + 10) + 90x}{x(x + 10)} = 3 \\[1em] \Rightarrow \dfrac{75x + 750 + 90x}{x^2 + 10x} = 3 \\[1em] \Rightarrow 750 + 165x = 3(x^2 + 10x) \\[1em] \Rightarrow 0 = 3x^2 + 30x - 165x - 750 \\[1em] \Rightarrow 3x^2 - 135x - 750 = 0 \\[1em] \Rightarrow 3(x^2 - 45x - 250) = 0 \\[1em] \Rightarrow x^2 - 45x - 250 = 0.

Hence, option (2) is the correct option.

3. Solving,

⇒ x2 - 45x - 250 = 0

⇒ x2 + 5x - 50x - 250 = 0

⇒ x(x + 5) - 50(x + 5) = 0

⇒ (x + 5)(x - 50) = 0

⇒ (x + 5) = 0 or (x - 50) = 0     [Using zero-product rule]

⇒ x = -5 or x = 50

⇒ x = 50 [As speed cannot be negative]

Speed = 50 km/hr

Hence, option (1) is the correct option.

4. The new speed of bus = x + 10 = 50 + 10 = 60 km/hr.

Hence, option (3) is the correct option.

5. Given,

Distance = 510 km

New speed = 60 km/hr.

Time = DistanceSpeed\dfrac{\text{Distance}}{\text{Speed}}

= 51060=812\dfrac{510}{60} = 8\dfrac{1}{2} hrs.

Hence, option (2) is the correct option.

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