subtract decimal hours from formatted hours

I need to subtract a number of hours in decimal format from a time formated as time

eg 180:30 (one hundred and eighty hours and a half hour time format with : )

less 162.5 (one hundred and sixty two and a half hours decmal format with . )

resulting in an answer of 18.0 (a decimal)

I cant work out how to change the 180:3 into 180.5
or how to change 162.5 into 162:30 automatically to apply to any different times.

Can someone help?
Many thanks

Thanks - that works to convert the time to a decimal.

E