Question

请问在1-100中有多少个“7”？

Answer 1

2,4,5,6,8,20,22,25,26,29,40,44,46,50,52,55,58,59,62,64,65,68,80,82,85,86,88,89,92,94,95,200,202,205,206,208,209,220,224,226,229,242,244,248,250,254,256,260,262,..

一种“拍7”的游戏规定:把从1起的自然数中含7的数称作“明7”,把7的倍数称作“暗7”,那么在1-100的自然数中,“明7”和“暗7”共有( )A.22个B.29个C.30个D.31个

[解]  明7一共有10+9=19个,7,17,27,37,47,57,67,77,87,97,70,71,72,73,74,75,76,78,79;暗7一共有14个,7,14,21,28,35,42,49,56,63,70,77,84,91,98,既是明7,又是暗7,3个,即7,70,77,∴共有19+14-3=30个.故选C.