The ratio of speed of two trains Z & X is 7 : 9.  If train Z takes 12 minutes more than train x to cover a certain distance , then how much time (in minutes) will X take to cover the same distance ?

42

 Distance = Speed ×  Time

So, ratio of speed to time is inverse to each other .

            Z   :   X

Speed   7   :   9

Time     9   :   7

According to question , 

 ( 9 - 7 )R = 2R = 12 minutes 

1R = 6 minutes

 So, Time taken by x →  7 × 6  = 42 minutes