At maximum speed an airplane travels 2100 miles against the wind in 6hrs. flying with the wind the plane can travel the same distance in 4hrs

Let, the speed of Airplane = x
Speed of Wind = y

We know, Speed = Distance / Time

So, x-y = 2100 / 6
x-y = 350
& x+y = 2100/4
x+y = 525

Substitute value of x from equation 1 into 2, 
350+y+y = 525
2y = 525-350
y = 175/2
y = 87.5

& x = 350+y = 437.5

In short, Speed of Airplane is 437.5 mph and Wind is 87.5 mph

Hope this helps!

Equation = $5x + $200= $450

1. $5x + $200= $450

2. $5x = $450 - $200

3. $5x = $250

4. x = ($250/$5)

5. x = 50 reams of paper