help!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!?

Question

i really tried this,
1. a train running at a uniform speed passes a bridge 275m long in 15 seconds and another bridge 425m long in 21 secs. find
1. the lenght of the train.
2. the speed of the train in km\hour.
ans=100m and 90km/hour

2. the speed of a boat in still water is 10km per hour. if the boat goes 37.5 kms upstream in 5hrs find the speed of the stream.
ans=2.5km/hour


3.the speed of a boat in still water is 9km/hr; if the boat goes 54 km down stream in 4hrs find the speed pf the stream.
ans=4.5km/hour

the formulae for speed is speed=distance/time.
hints for Q1. distance travelled by train= length of the train + length of the platform.
to convert km/hr to metre/ sec 5/18
and to convert metre/second to km/hr 18/5.

Q2. if the speed of a boat in the still water= x
and the speed of stream = y
then, the speed of boat in the irection opposite to the current(up stream)= x-y
and the speed of boat in the direction to the current = x+y.
pls help!

Answer 1

Let x = length of train
Let r = speed of train
Then r*15 = 275 +x or 15r-x = 275 <-- Eq 1
Also, r*21 = 425 +x or 21r -x = 425 <-- Eq 2
Subtract Eq 1 from Eq2 getting:
6r = 150
r= 25 m/s = 90 km/hour
Put r = 25 into Eq 2 getting 21*25 - 425 = x --> x = 100 meters

let x = speed of stream
then 10-x = speed of boat upstream
So (10-x)5 = =37.5
50-5x = 37.5
5x = 12.5
x = 2.5 km/hr

Let x = speed of stream
then 9+x = speed of boat downstream
So (9+x)4 = 54
36 + 4x = 54
4x = 18
x = 4.5 km/hr

Answer 2

actually u have got the ans in da hints only

iam qiute confused about Q1 but i know the other 2. they r very simple.

Q2. let the speed of stream be=x
speed of boat is given=10km/hr
therefore since the boat is goin upwards, its speed is =10-x
distance=37.5 km
time=5 hrs

so speed=distance /time
10-x=37.5/5
10-x=7.5
-x=7.5-10
-x=-2.5
multiplying both side with -1
therefore, x=2.5

similarly u can also do the next question. i know u have the capacity 2 solve it. all da best!!!!!!!!11

Answer 3

Use the formula
distance=velocity times time

let the length of the train be x
then
275+x=v . 15
425+x=v . 21
divide side by side
you will find
2125+5x=1925+7x
or x=100 m

Answer 4

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