How do I calculate time in excel

I am doing a spreadsheet for my boss for hours worked. For example I would enter 1.4 for 1hr and 40 minutes and the 1.2 for 1hr 20 minutes. I know it is backwards but they bill out in 10 minute increments. How do I get excel to add 1.4 and 1.2 so it equals 3 hrs.

Or for example 10 minute = 0.17 and 20 minutes = 0.33 but when excel adds 0.17+0.17 using the SUM formula it equals 0.34 instead of 0.33.
I'm frustrated and at a loss. Any advice would be appreciated.