This is a subtle math education question.
Consider this sequence of algebra steps:
a+b=c given
a=d given
d+b=c substitute
This is substitution. And feels natural to me. Replace a with d. But, I have many students who write:
a+b=c given
a=d given
b+d=c substitute
Still correct but I don’t understand why they swapped the order. What are they thinking? If it was just one or two students I’d call it “the noise of young minds” but it’s six of them. Why do they write it like this? #mathematic