Question 24360
Let the speed of the plane be x 
Let the speed of the wind be y


6h ---> the plane flies 2100km with a x speed against a y wind
5h ---> the plane returns with x speed with y wind 
against = - .  With = +
{{{2100/6(x-y)=2100/5(x+y)}}} ---> cross multiply.
5x+5y=2100 (1)
6x-6y=2100 (2)
Multiply (1) by 6
Multiply (2) by 5


30x+30y=12600 (ADD) --> eliminate y
30x-30y=10500 (ADD) --> eliminate y
-------------
60x = 23100
{{{60x/60=23100/60}}}
x = 385


Plug x into any equation 1 or 2.
5(385)+5y=2100
1925+5y=2100
5y = 175
{{{5y/5=175/5}}}
y = 35


Hence, the speed of the plane is 385mph and the speed of the wind is 35mph.
Paul.