You plan a car trip for which you want to average 90 km/h. You cover the first half of the distance at an average speed of only 48 km/h. What must your average speed be in the second half of the trip to meet your goal?

Question

anonymous · Answer

Oh wait. replace the 48 with 58. Sorry.

anonymous · Answer

guess we have to use $\frac{D}{T}=R$

anonymous · Answer

notice that one thing you are not told is the distance travelled
since you are not told, it obviously doesn't matter, so we have a choice, we can use the variable D to compute with or (what is almost always easier) is to pick a number, compute with that number, and whatever answer we get will be correct

anonymous · Answer

which would you prefer?

anonymous · Answer

Pick a number.

anonymous · Answer

ok i pick 100

anonymous · Answer

your time so far is $\frac{50}{58}$ i.e. distance divide by rate

anonymous · Answer

the rest of the time will be $\frac{50}{x}$ the rate you do not know

anonymous · Answer

and you must have the total distance divided by the total time be 90 i.e. 
$$\frac{100}{\frac{50}{58}+\frac{50}{x}}=90$$ solve for $x$

anonymous · Answer

you need help with that or are you good?

anonymous · Answer

I talked it over with some friends also doing this homework and we still dont get it... FYI This is Physics Homework.

anonymous · Answer

Oh wait we got it thanks anyway!