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1+9 is difficult to teach because usually we take greater number first. Hope I helped u.. Pls mark as the BRAINLIEST ans
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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....  :-)
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