Wouldn't hurt to know what was the procedure and what you were trying to determine (I guess I know, but I can be wrong).
Number of moles in 250cm3
= Same as 25cm3
Already looks suspicious, as in typical case we would take 25 mL out of 250 mL of the borax solution, so while the concentration is the same in both 250 mL and 25 mL, number of moles is not. Unless you diluted 25 mL to 250 mL, in which case yes, number of moles is identical - but the dilution is irrelevant.
Molar mass Na2B4O7·10H20 = 381.37 g/mol
How do you know it is 10H2
O? I thought it is up to you to determine this number?
You have correctly calculated mass of the anhydrous borax from the titration result, you know the initial mass of the sample. These two numbers allow easy calculation of mass of water present in the original sample.