one hour has 60 minutes not 100.
If you write 5.3 this means 5 hours and 0.3 hours which is 60 * 0.3 = 18 minutes.
If you want five-and-a-half hours you need to write 5.5 (0.5 is one half of 1 hour).
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Thank you so much @ScruffR for your kind help.