Có một bảng vuông 6 x 6 ô như hình vẽ. Người ta đặt 4 con chuột vào 4 ô vuông trong bảng, sao cho ô nào của bảng vuông cũng thẳng hàng theo hàng ngang, cột dọc hoặc đường chéo với một con chuột nào đó, nhưng các con chuột đều nằm trên các cột, các hàng và các đường chéo khác nhau. Các bạn hãy tính xem có bao nhiêu cách đặt các con chuột như vậy? Chú ý: Với một cách xếp, ta có thể quay hình theo góc vuông hoặc lật mặt để được thêm 7 cách xếp khác nữa, thành tất cả là 8 cách. Tuy nhiên 8 cách này không được tính là khác nhau mà chỉ được coi là 1 cách.

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Present, Past tense 1.I was the last to leave thee office.Everybody else..........(go) home 2.Laura was sitting on the ground. She was out of breath. She.....(run) 3.It's time we.....(do) something to stop road accident 4.I sat down at the desk and ......(wonder) why my father's letter......(not arrive) yet 5.I.......(make) a cake. That is why my hands were all over covered with flour 6.I came to the class late. When I.......(enter) the teacher(finish)..........going over the previous lesson and ......(write) something on the board 7.When I .......(open) the door last night I .......(see) a man on his knees.He clearly......(listen) to our conversation and I....(wonder) how much he ........(hear).When I .......(ask) him what he .......(do), he ........(say) he .........(drop) a 50 p piece outside the door and.......(look) for it 8.I never .......(see) anyone more punctual than your brother 9.My clothes aren't in the room.They....(take) them away 10."you......(be) here before?" "yes,I.....(spend) my holiday here last week" 11.Ms Abbott.....(walk) home from work when she....(see) a child fall of the bridge 12.Tell them to stop and take a rest.We....(work) long enough 13.You.....(find) the key which you.....(lose) yesterday?-Yes I ......(find) it in the packet of my other coat 14.He.....(be) to the theatre three times this month 15. It.......(rain) hard. We can't do anything until it ......(stop) 16.What.....(make) that terrible noise?It's the pneumatic drill.They.....(repair) the road. 17.The plane...........(fly) at 2,000 metres 18.The children are very quiet.Go and see what they.........(do).OH,they.......(cut) up some $5 notes 19.Why you .......(make) a cake?Someone......(come) to tea? 20.I......(do) some shopping yesterday,when I......(see) some my friends 21.Laura.....(miss) the party because no one....(tell)her about it 22.I.....(work) on the computer when the fire.....(break) out. 23.He........ (tell) us about his marriage when we.......(have) afternoon tea. 24.I .....(phone) you last night, but you.......(not answer).What you.....(do) at this time? 25.I.......(think) it's a pity you don't take more exercise.You.....(get) fat 26. While we .....(fish) someone........(come) to the house and......(leave) this note