Answer:
Molarity of NaOH solution is 1.009 M
Explanation:
Molar mass of HCl is 36.46 g/mol
Number moles = (mass)/(molar mass)
So, 0.8115 g of HCl =
HCl = 0.02226 moles HCl
1 mol of NaOH neutralizes 1 mol of HCl.
So, if molarity of NaOH solution is S(M) then moles of NaOH required to reach endpoint is 
So, 
or, S = 1.009
So, molarity of NaOH solution is 1.009 M
He could be blindfolded and know which was his and which was his sister's. All he would need to do is pick them both up and if they were too big then pick them up one at a time. The lumber might make it harder to tell, but this is a question about physical properties.
So there is a change in mass which for the purpose of this question should be quite different. His sister's ought to be much lighter than his. He would find it easier to pick up.