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# If the Speed of an Aeroplane is Reduced by 40 Km/Hr, It Takes 20 Minutes More to Cover 1200 Km. Find the Speed of the Aeroplane. - Mathematics

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Sum

If the speed of an aeroplane is reduced by 40 km/hr, it takes 20 minutes more to cover 1200 km. Find the speed of the aeroplane.

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#### Solution

Let the original speed of the aeroplane be x km/hr.

Time taken to cover a distance of 1200 km = 1200/x  hrs

("Time" = "Distance"/"Speed")

Let the new speed of the aeroplane be (x - 40) km/hr

Time taken to cover a distance of 1200 km = 1200/(x - 40) hrs

From the given information we have

1200/(x - 40) - 20/60 = 1200/x

1200/(x - 40) - 1200/x = 20/60

(1200x - 1200x + 48000)/(x(x - 40)) = 1/3

x(x - 40) = 48000 x 3

x^2 - 40x - 144000 = 0

x^2 - 400x + 360x - 144000 = 0

x(x - 400) + 360(x - 400) = 0

(x - 400)(x + 360) = 0

x = 400,-360

But, speed cannot be negative. So, x = 400.

Thus, the original speed of the aeroplane is 400 km/hr.

Concept: Quadratic Equations
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#### APPEARS IN

Selina Concise Maths Class 10 ICSE
Chapter 6 Solving (simple) Problems (Based on Quadratic Equations)
Exercise 6 (C) | Q 3 | Page 73

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