Let usual speed is Skm∕h. We know that, Time =
Distance
Speed
According to the question, ⇒
150
S−10
−
150
S
=
1
2
[Time =30min=
1
2
h ] ⇒
150S−150(S−10)
(S−10)S
=
1
2
⇒(150S−150S+1500)2 =(S2−10S) ⇒S2−10S−3000=0 ⇒S2−60S+50S−3000=0 ⇒S(S−60)+50(S−60)=0 ⇒(S+50)(S−60)=0 S=−50,S=60 Speed can't be negative. So, speed =60km∕h Now, if distance =90km, then drive he 15km∕h slower =60−15 =45km∕h. ∴ Time =