Let the original average speed of the train = x km/h According to the question,
63
x
+
72
x+6
=3
{time=
distance
speed
}
⇒ 9[
7
x
+
8
x+6
]=3 ⇒ 3[
7x+42+8x
x2+6x
]=1 ⇒ 45x+126+=x2+6x ⇒ x2−39x−126=0 ⇒ x2−42x+3x−126=0 ⇒x(x−42)+3(x−42)=0 ⇒x(x−42)(x+3)=0 ∴x=42,−3 Hence, the original average speed of the train is 42 km/h.