A train covers 240 km at a uniform speed. If its speed is increased by 4 km/h, it takes 0.5 hour less. What is the original speed?

Explanation

Time at original speed = 240 / x  

Time at increased speed = 240 / (x + 4)  

Given:  

240/x - 240/(x + 4) = 0.5

Multiply through by x(x + 4) to eliminate denominators:

240(x + 4) - 240x = 0.5x(x + 4)  

240x + 960 - 240x = 0.5x² + 2x  

960 = 0.5x² + 2x  

Multiply both sides by 2:  

1920 = x² + 4x  

x² + 4x - 1920 = 0

Solve the quadratic:  

x² + 4x - 1920 = 0  

Using quadratic formula:  

x = [-4 ± √(16 + 7680)] / 2  

x = [-4 ± √7696] / 2  

√7696 ≈ 87.7  

x ≈ (-4 + 87.7)/2 ≈ 83.7/2 ≈ 41.85

Approximate correct option is 40 km/h

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