[질문] 의합이 21의배수인것은 모두몇쌍인지 구하시오.

14쌍입니다.(6,7,8)(13,14,15)(20,21,22)(27,28,29)(34,35,36)(41,42,43)(48,49,50)(55,56,57)(62,63,64)(69,70,71)(76,77,78)(83,84,85)(90,91,92)(97,98,99)