Question 807785
Let {{{ w }}}  = the rate of the wind
Let {{{ b }}} = the rate of the bird in still air
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Going with the wind:
{{{ b + w = 40 }}} km/hr
Going against the wind:
{{{ b - w = 10 }}} km/hr
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Add the equations
{{{ 2b - 50 }}}
{{{ b = 25 }}}
and
{{{ b + w = 40 }}} 
{{{ 25 + w = 40 }}} 
{{{ w = 15 }}}
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The rate of the wind is 15 km/hr
The rate of the bird in still air is 25 km/hr