A is twice fast as B and B is thrice fast as C. The journey covered by C in 78 minutes will be covered by A in :

Explanation:-

The ratio of speeds of A, B, C = 6 : 3 : 1.
The ratio of time taken by A, B, C = (¹/₆) : (¹/₃) : (1)
To simplify it, we will multiply it by LCM of ratio of speeds given.
Hence, the ratio of time taken by A, B, C = 1 : 2 : 6.
[Speed is inversely proportional to time, means if speed increase time decreases. So, ratio of time would be reciprocal of the ratio of speed given. ]
Time taken by C to covered given distance = 78 = 6 * 13.
The ratio of time of A and C = 1 : 6
Thus, time taken by A = 13 min.