how to subtract two strings in sql server

Question

how to subtract two strings in sql 

9.15-1.15=8 

i have a one table called sample,in that table i have a column name is Workedhours

 

 workedhours

 9.15

 10.20

 11.50

my expected result is subtract the workedhours 1.15

like 9.15-1.15

10.20-1.15

 workedhours

 8

 9.05