0---9十个数字中选3个组成3位数,这样组成的三位数有几个,大于800的有个几?

1.10*9*8=720(个） 2.2*9*8=144（个）

987,986,985,984,983,982,981,980,978,976,975,974,973,972,971,970,968,967,965,964,963,962,961,960,958,957,956,954,953,952,951,950,948,947,946,945,943,942,941,940,938,937,936,935,934,932,931,930,928,927,926,925,924,923,921,920,918,917,916,915,914,913,912,910..........共576个,大与800的有128个

8*8=64
64*3*2=384

2*8=16
16*2=32