Question

the speed of an aeroplane is reduced by 40 km per hour it takes 20 minutes more to cover 1200 find the speed of the aeroplane

Answer 1

Let the actual speed of the aeroplane be 'x' km/hr

So, speed after reduction = (x - 40) km/hr

Let the actual time taken by the aeroplane be y hr

and the time taken after the speed reduction = y + 20 min = (y + 20/60)

= (y + 1/3) hr

Total distance = 1200 km

So, 

Distance = speed*time

1200 = x*y 

xy = 1200

y = 1200/x .............(1)

After speed reduction,

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

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

1200*3 = (x -40) (3y + 1)

Now, substituting the value of y = 1200/x, we get 

3600 = (x - 40) (3*1200/x + 1)

3600 = (x - 40) (3600/x + 1)

3600x = (x - 40) (3600 + x)

3600x = x² + 3600x - 40x - 144000

x² - 40x - 144000 = 0

x² - 400x +360x - 144000 = 0

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

(x - 400) (x + 360)

x = 400 and x = -360

Speed can't be negative, so the actual speed of the aeroplane is 400 km/hr

substituting the value of y = 1200/x.

y = 1200/400

y = 3 hours

So, the actual time taken by the aeroplane is 3 hours.

Answer 2

1200 MTR in 20 min
in 60 mint 3600mtr
speed is 3.6 km/h