It is difficult to teach (1+9).
This is because
for (9+1), One knows the number 9 already and also knows number 1. So the child has to add just one number to 9.
But when the child has to add 9 to 1 , He has to add the number 1 nine times again as till now he is not aware of the number 9.
Because 1 is taught first and 9 later.[going in serial order]
so being aware of 9 and 1 (9+1) is easy but being unaware of 9, (1+9) is difficult even to teach and even to calculate.
and mark it as 'BRAINLIEST" if it deserves.... :-)