Thks everyone for yr replies and help. Fyi, the one that posted
this qn had corrected and reposted the qn as below :
A wave of the water current was
flowing at a speed of 8 km/h towards Point X from Point Y which was
865 km apart. At 8 am, Johnny rode off with his boat at a speed of
50 km/h towards Y from Point X for 30 mins and then stopped to have
a rest for 45 mins before continuing his remaining journey at the
same speed. At 9.15 am, Peter set off with his boat at a speed of
120 km/h from Point Y to Point X and both of them travelled for a
distance and finally met at Point Z. How long did Johnny take to
travel after passing Peter at Point Z? Express your answer in hrs.
[ 5 Marks ]
Accordingly to him/her, the answer is still 6 1/4 h and this qn
has nothing to do with relative
velocity.
Any comments and Thks.
I still think the question is incorrect. It should be rephrased
as how long it takes for Johnny to meet Peter at point Z from the
time he starts off from point X.
Assuming that the point I made above is correct, the workings
will be as followed:
Distance travelled by Johnny in half an hour
= 1/2 X 50km - 1/2 X 8km
= 21km
While resting for 45mins, distance the boat drifted back
= 3/4 X 8km
= 6km
As such, after 1h 15mins, actual distance travelled is only
= 21km - 6km
=15km
Distance between Johnny now and Peter
= 865km - 15km
=850km
Actual travelling speed of Johnny
= 50km/h - 8km/h
= 42km/h
Actual travelling speed of Peter
= 120km/h + 8km/h
= 128km/h
Time taken for the two guys to meet each other
= 850km / ( 42km + 128km )
= 850/170 hours
= 5 hours
Total time for the 2 guys to meet
= 5 hours + 45 mins + 30 mins
= 6 and
1/4 hours.
If the question is correct whereas the one I proposed is wrong,
the final answer will be over 15 hours, not to mention with some
decimal places.