sqrt2 /10

Do I just leave this as sqrt2/10 or can I reduce it to sqrt1/5?

I don't know what the original question is. If the answer given is sqrt(2/10), most teachers expect the answer to be rationalized, namely, no square-root sign in the denominator.

So
sqrt(2/10)
=sqrt(1/5)
=1/sqrt(5)
=sqrt(5)/sqrt(5)²
=sqrt(5)/5