if the number is xyz
z= x+y
max value of z=9 to satisfy the given condition, x can take any value from 0to 9, but y takes only one value for a given x,i.e 9-x
so there are 9 possibilities for z=9
similarily for z=8,there are 8 possibilities and so on
so required combination is 9+8+7+6+.....+1=9*10/2=45 numbers