> 
> * All of these factors make them an ideal population in which to test these competing hypotheses about how language is learned.
> 
> 
> 

in which to?

> 
> * These children began learning English at an older age than Us natives and had more mature brains with which to tackle the test.
> 
> 
> 

with which to?

Can't we just omit "in which" and "with which"?

> 
> * All of these factors make them an ideal population to test these competing hypotheses about how language is learned.
> * These children began learning English at an older age than Us natives and had more mature brains to tackle the test.
> 
> 
> 

Can somebody please explain the grammar here?

Just to be sure, can we we bring "to + simple verb" after "in which" and "at which"?