an express train starts from chennai at a speed of 50 km/hr. after one hour an another express starts at a speed of 70 km/hr. after how many hours will the second express train cross the first?
please reply me with the correct answer and steps.
2007-01-07 19:10:32 · 18 answers · asked by krithika R 1 in Science & Mathematics ➔ Mathematics
THE OPTIONS ARE:
a) 3 hours b) 2 3/4 hrs c) 3 1/2 hrs d) 2 1/4 hrs e) none of these
2007-01-07 19:18:45 · update #1
After 1 hour, the first train is 50km away.
An expression for the distance of the first train from Chennai after that time:
x = 50 +50t
An expression for the distance of the second train from Chennai after that time:
y = 70t
Set the two equations equal to eachother:
x = y
50 +50t = 70t
20t = 50
t = 5/2 hours = 2 hours 30 minutes.
2007-01-07 19:14:31 · answer #1 · answered by Patrick M 2 · 2⤊ 1⤋
Relative speed = 20 km/h { 70-50}
Total difference between 50 km {distance covered by first train before second train started }
time = 50/20 = 2 h 30 min
answer is 2.5 hours
2007-01-07 19:38:52 · answer #2 · answered by Ritesh13171 3 · 0⤊ 0⤋
I know it's not the proper way to figure these out but it's never failed me before. The first train is going 50 km/h, it leaves an hour before the second. Therefore is has a 50 km lead. The second train leave and is going 70 km/h, going 20 km/h faster than the first. Therefore its gaining 20 km/h. The first train has a 50 km lead, 50 divided by 20 equals 2.5 hours.
The answer is 2.5 or 2 1/2 hours.
2007-01-07 19:15:34 · answer #3 · answered by Phoenix 3 · 1⤊ 0⤋
Another approach
Let x be the distance they meet
2nd train will travel the distance in x/70 hours
1st train will travel with head start of 50km
hence to reach the same point it will take(x-50)/50 hrs
equating these
(x/70)=(x-50)/50
solve for x
x = 175 km
time taken by 2nd train to reach the 1st train
=175/70 = 2hrs,30minutes.
2007-01-07 19:47:58 · answer #4 · answered by drsgp 1 · 0⤊ 0⤋
50(t + 1 hr) = 70t
20t = 50 hr
t = 50/20 hr = 5/2 hr = 2 hr 30 min
2007-01-07 19:42:30 · answer #5 · answered by Helmut 7 · 0⤊ 0⤋
Distance traveled by 1st train: 50+50t where t=time in hours;
" " by 2nd train: 70t;
When the total distance is the same:
70t = 50 + 50t; or
70t - 50t = 50; or
20t = 50; therefore
t = 50/20 = 5/2 = 2.5 hours.
You can double check by plugging 2.5 as t into both expressions:
50t+50 and 70t.
2007-01-07 19:20:05 · answer #6 · answered by kyq 2 · 1⤊ 0⤋
Let it be "t" h.
Distance travelled by both trains:
1st Train: 50xt+50 (50 km covered in 1 h)
2nd train: 70xt
Therefore:
50t+50=70t
Solve for "t": t=2.5 h.
Means after 2.5 h (after the departure of 2nd train), the 2nd train would be about to cross the first train. Secondly to cross completely, you ought to know the length of both the trains.
2007-01-07 19:20:33 · answer #7 · answered by Raja Ram 1 · 0⤊ 0⤋
first train distance = 50km/hr *(t+1)
second train distance = 70km/hr *t
50t+50 = 70t
20t = 50
t = 5/2 hours
=2.5 hours.
2007-01-07 19:21:35 · answer #8 · answered by beanie_boy_007 3 · 0⤊ 0⤋
this is a simple time distance problem
soln:
let 2nd train crosses the 1st in t hrs.
then distance travelled by 2nd train =70*t km
and distance travelled by 1st train=50*(t+1) km
therefore
since distance travelled is same
70*t=50*(t+1)
hence t=2.5 hrs
so option e is correct
2007-01-07 19:47:45 · answer #9 · answered by viv34207 1 · 0⤊ 0⤋
yes the secondtrain start after ahour so
t1travel at 2 hour =100km
t2travel at 2 hour= 70 km because it start after one hour
then t1 at 2.5 hour = 125km because its speed is 50 km /h
for half hour is 25 .
then t2 travels at 2.5 hour=105 km because it can trvel 35km for half hour and so 70+35=105
then at 3rd hour
t1travel 150km
t2 travel passed 140km
so secondtrin over take train1 at 3rd hour
so answr should be 3rd hour
2007-01-10 20:07:10 · answer #10 · answered by fervej 2 · 0⤊ 0⤋