Explanation:

Given, Ratio of speed of trains = 6 : 7
Second train covers 364 kms in 4 hours
Then, its speed = 364/4

 = 91 km/hr
In the question it is given that speed of the second train = 7 units
But actual speed = 91 km/hr
i.e., 7 units → 91 km
1 unit → 13 km
Therefore,
Speed of the first train is :
= 6R
= 6 × 13
= 78 km/hr