Problema Solution
A woman finds that if she travels 5 kilometers per hour faster than her normal speed, the lenght of time it takes her to make a trip of 70 kilometers can be decreased by 15 minutes. What is her normal speed?
Answer provided by our tutors
Let x be the normal speed.
15 minutes =15/60 =1/4H
Then 70/x-70/(x+5)=1/4.
70(1/x-1/(x+5))=1/4.
1/x-1/(x+5)=1/280.
(x+5-x)/x*(x+5)=1/280.
5/(x^2+5x)=1/280.
x^2+5x=280*5.
x^2+5x-1400=0.
By factorisation
(x-35)(x+40)=0
x=35,-40.
Normal speed =35 kmph.