Any help would be appreciated!

We have   24 total  members....this is the "population'

The 10 members of the group  would represent a "sample" of the population

So...we just sum the hours of sleep and divide by 10 to get the  sample mean  .....so we have

[ 8 + 8 + 8.5 +  8 + 9  + 8 + 9 + 9 + 8.5 + 8  ]  / 10  =   8.4 h   = the sample mean