Question 32118
Let the average speed be x
When it is decreased it will be: x-100
and time increased by 30 minutes or 0.5 hours:
TOtal distance of flying wil respect to speed and time = 2800
Equation:
*[tex \frac{2800}{x}-\frac{2800}{x-100}=0.5]
<pre><i>Cross multiply</i><b>
2800[(x)-(x-100)]=0.5[(x)(x-100)]
{{{2800(100)=0.5(x^2-100x)}}}
{{{0.5x^2-50x-280000}}}
Multiply the eqauiton by 2 to remove the decimal:
{{{x^2-100x-560000=0}}}
*[tex x = \frac{-b \frac{+}{-} sqrt( b^2-4*a*c )}{(2*a)}] 
In quadratic equation: a=1, b=-100, c=-560000
*[tex x=\frac{-(-100)\frac{+}{-}sqrt((-100)^2-4*1*-560000)}{2*1}]
Simplfy 
x=-700 or x=800
<font color = red>Hence, the average speed was 800km/h.</font>
Paul.