请教一数学题黄皮OG

chensishui

Of the three-digit integers greater than 700,how many have two tigit that are equal to each other and the remaing gigit different from the other two?

 A 90

B 82

C 80

D 45

E 36

gan4sula

80個, 700如果不算在內
701~999
707, 717...797, 9個, 777沒算
770,771,...779, 9, 777 沒算
711,722...799, 8, 700 and 777 沒算

800 號的也有 9+9+9(811-899 只有888沒算)
900號的也是

(9+9+9)x3 -1(這個是700)
=80

[此贴子已经被作者于2008-4-14 12:38:41编辑过]

mandy_lulu

我的理解是按组合算的

百位数只有７，８，９　那如果同百位的就是　Ｃ（１，３）Ｃ（１，１０）Ｐ（２，２）＝６０

如果不同百位就是个位和十位相同，Ｃ（１，１０）Ｐ（２，２）＝２０

相加就是８０