og ds # 69

[simba12123]

athlete runs R miles in H hours, then rides a bike Q miles in same number of hours. WHich of following represents the athelestes average speed, in miles per hour, for these two activities combined?

a. (R-Q)/h
b.(R-Q)/2h
c.2(r-q)/h
d.2(r-q)/sh
e.(r+q)/2h


qa is e i got d..

plugging in numbers with r=20 h=5 and q =20

total distance over total time. I got 40 miles over (4 plus 1 hour) this yields a time of 8 as target number to look for in answer choice. d is good but not qa.. whats up?

[cramya]

You are right about
Average speed = Total distance travelled / Total Time taken

plugging in numbers with r=20 h=5 and q =20

h=5 again for Q miles

Total Distance travelled = R+Q

Total time taken = H+H = 2H

Therfore average speed = R+Q/2H which is E)

Using your numbers it will be 20+20/2*5 = 4 miles per hour

For average speed I dont think u can divide the first 20/5 and the next 20/5 and add them together to get 8.

You would have to add the total distance travelled / total time taken.

P.S:

For examle there may be situations where you will be given 2 speeds and asked to find the average speed. In this case you make up a number for distance lets say the lcm of 2 speeds given and then find the number of hours taken with each speed. Then add the 2 equal distances and divide by the total time to get average speed.

Hope this helps!

[vishubn]

It works !! even with the values R=10 and Q=20 and the h=2

he Runs 10 miles in two hours
he rides 20 miles in 2 hours again
so total distance covered is 30 miles in 4 hours !

so speed is 7.5 miles/hr

so put the value back in choices


PostPosted: Sat Oct 18, 2008 8:18 am Post subject: og ds # 69
athlete runs R miles in H hours, then rides a bike Q miles in same number of hours. WHich of following represents the athelestes average speed, in miles per hour, for these two activities combined?

a. (R-Q)/h=-ve value so ruled out
b.(R-Q)/2h=same as above
c.2(r-q)/h=same as above
d.2(r-q)/sh=same as above
e.(r+q)/2h= 7.5 and satisfies the condition

VIshu

OA =E