convert assumption to weaken

[AsadA969]

My brother bought a fair of shoes yesterday in size 13. He usually wears size 11. So, he bought the wrong size.
 Possible challenge: What if he bought a brand that run smalls?
 Assumption: The brand he bought does not run significantly smaller than his usual brands.
 Weaken: The brand he bought significantly smaller than his usual brands.
 Possible challenge: What if he bought the shoes for someone else?
 Assumption: He bought the shows for himself.
 Weaken: He bought the shoes for someone else rather than himself.
If I make any assumption false, will it be ‘weaken’ like the above?

[RonPurewal]

the reasoning in those examples is valid -- but i can't tell what your question is.

please clarify, thanks.

[AsadA969]

My brother bought a fair of shoes yesterday in size 13. He usually wears size 11. So, he bought the wrong size.
 Possible challenge: What if he bought a brand that run smalls?
 Assumption: The brand he bought does not run significantly smaller than his usual brands.
 Weaken: The brand he bought significantly smaller than his usual brands.
 Possible challenge: What if he bought the shoes for someone else?
 Assumption: He bought the shows for himself.
 Weaken: He bought the shoes for someone else rather than himself.
If I make any assumption false, will it be ‘weaken’ like the above?

Does the below weaken the argument (My brother bought a fair........)?
Weaken:
1) The brand he bought significantly smaller than his usual brands.
2) He bought the shoes for someone else rather than himself.
Thanks...

[RonPurewal]

yes.