@zabeer agreed bro. But go through this thread. There are 5-6 more wrong questions. And I have few friends who actually believed the solution. Really appreciate the collection and greatful for it, but many people would follow solutions as it is. Anger is on the part that a friend who showed this actually did same mistake in mock saying that he learnt the solution from here.
Hope you understand. No offense.

@vikas_saini can u plz explain 2nd and 3rd+4th step again. 2nd step:what is -1-what are we reducing in it
3rd step: 3rd+4th step doesnt giv value in 2nd step. what are we missing in it. 3rd+4th= 2T-2

a= speed of train starting from P
b= speed of train starting from Q
PQ/QR=a/b
(PQ+QR)/QR =(a+b)/b
QR=PR.b/(a+b)
QR/a=16
PR.b/((a+b).a)=16 ....(1)
Similarly PR.b/((a+b).a)=25 ....(2)
dividing 1 by 2
16a/b=25b/a
(a.a)/(b.b)=25/16
a/b=5/4
(a+b)/3=675
a+b=225
b=100
a=125
from 1
PR/a=16*(a+b)/b
=16*225/100
=36hr
so ans is b

@master_shifu
Can we not calculate the average speed by adding the total distance, i.e 9000 kms and then adding the total time taken , i.e. 25 hours + 24 hours + 1.55(halt in India) and then dividing those figures. Total distance/total time?

@shreyasnegi13
There are 10 primes in the given range.
4 also satisfies
9 and 25 won't work because we would already have two 3s and 5s respectively in the numerator
So 10 + 1 = 11 values.

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