a bird flying with the usual wind travels 50km hr but when flying against the wind that is twice as strong as the ususal oneits only 20km/hr what is the speed of the bird in air and the ususal speed of wind

Answer:
Let speed of bird = x  km / hr
Speed of wind  = y km / hr
And
we know 
$Speed = \frac{Dis\tan ce}{Time}$

So from first condition

x  + y  = 50      ------- ( 1 )             ( As bird travels 50 km in a hr , when flying with the wind )

And

x  - 2y  = 20  ----------- ( 2 )          ( AS bird flying against the wind that is twice as strong as usual )

Now subtract equation 1 and 2 , we get

3y  = 30
y = 10             ( Substitute that value in equation 1 we get )

x + 10 = 50

x = 40 

So,
The speed of the bird = 40 km / hr

The usual speed of wind = 10 km /hr                               ( Ans )