Answer
89.992$\leq$d$\leq$90.008
Work Step by Step
Let d be the actual diameter of a piston in mm
The difference between the actual & ideal diameter is at most 0.008
|d-90000|$\leq$0.008 => d-90000$\leq$0.008 and d-90000$\geq$-0.008 => d $\leq$90000.008 and d$\geq$89999.992
or 89999.992$\leq$d$\leq$90000.008