Let the original average speed (S1)=x Average speed reduced by 100(S2)=x−100 Total speed ∆S=S2−S1=x−x−100=100 ⇒D=
S1×S2
∆S
×∆t ⇒2800=
x(x−100)
100
×
1
2
⇒560000=x(x−100) ⇒x2−100x−560000=0 ⇒x2−800x+700x−560000=0 ⇒x(x−800)+700(x−800)=0 ⇒(x−800)(x+700)=0 ⇒x=800 or x=−700 ∴ The original average speed of the aircraft is 800km∕hr.