danny drove halfway from town p to town q at average speed of 60 km/h. he wants to average 80 km/h for the entire journey. What will his average speed be for the second part of his journey?

Answer 1

speed=distance/time

60=d/2time1

but d=60*time1+V*time2

and
80=d/(time1+time2)
or
time1+time2=d/80 then
d/120+time2=d/80

time2=d/80-d/120=d/120 (3/2-1)
time2=d/240

so velocity2ndpart=(d/2)/time2
= d/2(d/240)=120km/hr

check:
time1=d/(2*60)=d/120
time2=d/(2*120)=d/240

80=d/(time1+time2)=d/(3d/240)=80