Look at the shape: three 十 (ten) joined at the base, so 'three tens' = thirty. Compare 廿 (twenty, two tens). Most famous in 五卅 (May 30th).
Components
ideograph
卅sà
thirty (three tens joined)
卅 is an abstract numeral made by fusing three 十 (ten) signs into one connected shape, directly writing 'thirty'. It is not built from smaller independent parts, so it stands as a single unit.
Filed under radical 十 (shí, #24) by convention. 十 is not a separate component in 卅, so no strokes are highlighted.
