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abhiraj77

· started a discussion

· 1 Months ago

easy solution (Question: 99439.0 )

time taken T1 to complete 75% distance = 3d/4s
and time taken T2 to cover 25 % (if speed is not twiced) = d/4s
now
T1-T2=3hours
T= 6Hours

Question:
Amy starts in a car from Ahmedabad towards Bangalore. After sometime she realizes that he will cover only 75% of the distance in the scheduled time and she therefore doubles his speed immediately and thus manages to reach Bangalore exactly on time. Find the time after which Amy changed his speed, given that he could have been late by 3 hours if he had not changed his speed.
Options:
A) 3 hour
B) 4 hour
C) 5 hour
D) 6 hour
E) None of these
Solution:
Let the original speed be s km/h and scheduled time = t hours And total distance = D km 
Then s × t = ¾ D …(i) 
And s × (t + 3) = D …(ii) 
From Eq. (i) and (ii), we get st = ¾ [s(t + 3)] ⇒ t = 9h 
and let s = 1 km/h, then D = 12km 
Again, since he doubles his speed after k hours then, 
s1t1 + s2t2 = D 
1 × k +2x(9- k) =12 
⇒ k = 6h

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