Question 1020035
A man jogged 3.5 km/h in 20 min.
   He then walked the remainder of the 5.5 km at an average speed of 5 km/h.
 Given that he did not stop to rest during the entire distance of 5.5 km, calculate his average speed in km/h for the Whole journey.
:
Let a = his average speed of the journey
:
Change 20 min to 1/3 hrs
:
Write a time equation; time = dist/speed
1st 3.5 km time + remaining 2km time = total time
{{{1/3}}} + {{{2/5}}} = {{{5.5/a}}}
multiply by 15a
15a*{{{1/3}}} + 15a*{{{2/5}}} = 15a{{{5.5/a}}}
Cancel the denominators and you have
5a + 3a(2) = 15(5.5)
5a + 6a  = 82.5
11a = 82.5
 a = 82.5/11
 a = 7.5 km/hr is the average speed of the whole trip