Sorry can't answer that question for you.
By bus. Easy. Go to the main bus-station. Frequently buses to Opatija and further to Lovran.
From rijeka you need to take Bus #32 and it will write Opatija. The ride is about 10-15min. On stand you need to buy bus ticket and say for Opatija and they will give you return one (zone 3), I think it cost about 32KN or arround 5€. The best way to go on bus in Rijeka is on Jelacic square and accros the road you will see bus parking and from there bus 32 goes for Opatija.
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