15 = 1+1+13 = 1+2+12 = 1+3+11 = 1+4+10 = 1+5+9 = 1+6+8 = 1+7+7= 2+2+11 = 2+3+10 = 2+4+9 = 2+5+8 = 2+6+7= 3+3+9 = 3+4+8 = 3+5+7 = 3+6+6= 4+4+7 = 4+5+6= 5+5+5共19种.