The speed of boat 'X' in upstream is 18 km/hr and speed of boat 'Y' in downstream is 30 km/hr. If the difference between speeds of boat 'X' and 'Y' in still water is 20 km/hr, then find the time taken by boat 'X' to cover 210 kilometres in downstream.
(Note: speed of boat 'X' in still water is more than that of boat 'Y' and both boats are travelling in same river)