This problem is only asking for the total number of hours he worked. So to begin, as this is an addition problem, you would take the numbers and put them in a sequence like the one below:
6.23 + 7.45 + 8.3 + 8 + 5.21
Then you would add the numbers together, remembering the decimals out of 100, and the total sum would be your answer. Answer: 35.19 hours in total. Hope this helped you :)!