PSLEguru:

HengTing:

[quote=\"PSLEguru\"]
Method 3
They met 45km from midpoint. So Tan must have travelled 45*2 = 90km more than Lim.
Tan is travelling at 90-65 = 25km/hr faster than lim.
To travel 90km more than Lim, he would take 90/25 = 3.6hr.
So total distance = 3.6 * (90+65) = 558km

Hi need help to explain the Method 3 to my kid,.
To travel 90km more than Lim, he would take 90/25 = 3.6hr. -- why need to divide by 25? and get 3.6hr is meant for total distance = 3.6 * (90+65) = 558km -->why need to add Mr Lim & Mr Tan speed ?
Thanks in advance

Hi,
Hope this helps

If A is travelling 10km/h faster than B. After 1 hr, A has travelled 10km further than B.
In this case, Tan is travelling 90km more than Lim, and he is travelling at 25km/h faster. How long will he take? He will take 90/25 = 3.6hr.

In 3.6hr Tan would have travelled 3.6*90km.
In 3.6hr Lim would have travelled 3.6*65km.
Together, they would have travelled 3.6*(90+65) = 558km[/quote]Thank you for explanation.. :thankyou:

PSLEguru:

PSLEguru:

[quote=\"Mary Joy\"]Pls help me with this speed question...tia


http://i44.tinypic.com/34zgi0z.jpg\">

Method 1: algebra
X ------------|-------|-------------------Y
lim(65)------|45km|----------------Tan(90)

let distance be 2x. so X to midpoint = Y to midpoint = x.
since they left at the same time
(x+45)/90 = (x-45)/60
(x+45)/18 = (x-45)/13
13x+585 = 18x-810
5x = 1395
x = 279
distance = 2x = 558km.

Method 2: suppose
combined speed = 90+65 = 155km/hr.
suppose distance between X and Y is 155km.
They would meet after 1hr. i.e. lim travelled 65 and Tan travelled 90.
midpoint is 155/2 = 77.5.
distance from midpoint = 90-77.5 = 12.5km
they would have met 12.5km from the midpoint.

we know that they met 45km from the mid point. so the distance (and time taken) must be greater. How much greater?
45/12.5 = 3.6 times greater.
distance = 3.6 * 155 = 558km.

Method 3
They met 45km from midpoint. So Tan must have travelled 45*2 = 90km more than Lim.
Tan is travelling at 90-65 = 25km/hr faster than lim.
To travel 90km more than Lim, he would take 90/25 = 3.6hr.
So total distance = 3.6 * (90+65) = 558km[/quote]Hi need help to explain the Method 3 to my kid,.
To travel 90km more than Lim, he would take 90/25 = 3.6hr. -- why need to divide by 25? and get 3.6hr is meant for total distance = 3.6 * (90+65) = 558km -->why need to add Mr Lim & Mr Tan speed ?
Thanks in advance