Train A travelling at 'S' m/sec. , can cross a plateform double its length in '36' seconds, the same train travelling at (S + 3 ) m/sec. can cross the same plateform in '15' seconds. What is the value of 'S' ?

By formula-

Speed = Distance / Time

STEP 1 :

Putting the values -

S= 3x / 36

S = 1 / 12

12S = x

STEP 2 :

S + 3 = 3 * 12S / 15

S + 3 = 36S / 15

5 ( S + 3 ) = 12 S

5S + 15 = 12 S

15 = 12 S - 5S

15 = 7S

2.1428571428571 m/sec. = S

S = 2.1428571428571 m/sec.