***7 different letters, the same letter is A R N E***4 letters, and the non-repeated letters are 3 letters.

1.

If you choose four different letters, then C (7 7,4) =, there are 35 kinds, so combine them. 35*4*3*2=840 species.

If one of the four letters is repeated, repeat one of the four identical letters, C(4, 1), and then take two of the remaining six letters. C (6,2) * * has C (4,1) * c (6,2) = 4 *.

60*4*3*2/2=720

If there are only two letters in four letters, choose from four repeated letters, and c (4,2) = 6; If two identical letters are connected together, there are two kinds; If the same letters are not connected together, there are four kinds, and * * has 6*(2+4)=36.

So * * * has 840+720+36= 1596 species.

2.

If there are two r's in four letters, you can see the arrangement of other letters.

If other letters are different, then C (7 7,2) = 21,* * has 2 1*4*3*2/2=252 kinds.

If the other two letters are the same, choose one of the remaining A N E, c (3, 1) = 3, and * * has 3*(2+4)= 18.

So * * * has 252+ 18=270 kinds.

3.

When two r's are arranged together, four letters are combined into three letters.

If the remaining letters are different, then c (7,2) * 3 * 2 =126.

If the remaining letters are the same, then C(3, 1)*3=9.

* * * There are 126+9= 135 species.