解：?$ (1)$?對(duì)折?$2$?次后，厚度是?$22×0.1=0.4(\mathrm {mm}) ；$?
?$(2)$?由題意可知：對(duì)折?$n$?次，厚度為?$ 2^{n}×0.1(\mathrm {mm}).$?
對(duì)折?$20$?次后，厚度是?$ 2^{20}×0.1=104857.6(\mathrm {mm}) ；$?
?$(3 ) 3×1000×30=90000($?毫米?$)$?
?$104857.6>90000$?
答：對(duì)折?$20$?次后有?$30$?層樓高?$.$?
