1到100的质数为:2.3.5.7.11.13.17.19.23.29.31.37.41.43.47.53.59.61.67.71.73.79.83.89.97;
693可以分解为:3×3×7×11,
3×7-6=15不是质数,3×11-6=27不是质数,3×3×7-6=57不是质数,3×3×11-6=93不是质数,3×3×11-6=93不是质数,只有3×3-6=3是质数,只有7×11-6=71是质数,故只能是爷爷77岁,孙子9岁.
1到100的质数为:2.3.5.7.11.13.17.19.23.29.31.37.41.43.47.53.59.61.67.71.73.79.83.89.97;
693可以分解为:3×3×7×11,
3×7-6=15不是质数,3×11-6=27不是质数,3×3×7-6=57不是质数,3×3×11-6=93不是质数,3×3×11-6=93不是质数,只有3×3-6=3是质数,只有7×11-6=71是质数,故只能是爷爷77岁,孙子9岁.