I think this is more of an opinion question. What someone could solve mentally may not be the same as someone else's.If it were my worksheet, I'd circle all of them but it may be different for you.
At your level, I'd probably circle:
54 - 10
93 - 20
and 39 - 2
You need to win 30 matches in a row to raise its winning percentage to 90%
6n-8<5n decreased means subtract
n-8<0
n<8