The rule associated with moving decimal places, and multiplying by a value or multiple of 10, would be the following :
If you divide a value by 10 or a multiple of 10, you would move the decimal place to the left number of times as there are zeros in the multiple.
(Assuming that there are no zeros in the numerator value)
If you multiply by 10 multiple, then you would move the decimal to the right that many times as there are 0's in the value.
The solution would be that the decimal number is multiplied by 100.
Visualize one train going north and the other going south. To determine their distance apart, we could add the distance one train travels north to the distance that the other train travels south.
Thus, |62 kph * t| + |68 kph * t] = 455 km (Seems that you may be mixing km with miles; can't do that; choose one or the other.)
Then
455 km
------------------------ = t = 3.5 hours
(62 kph + 68 kph)