The ML of 0.85 m NaOH required to titrate 25 ml of 0.72m hbr to the equivalence point is calculated as follows
calculate the moles of HBr used
moles = molarity x volume
25 x0.072/1000= 0.0018 moles
write the equation for reaction
NaOH + HBr = NaBr + H2O
from reacting equation the mole ratio between NaOH to HBr is 1:1 therefore the moles of NaOH = 0.0018 moles
volume = moles/molarity
0.0018/0.085 = 0.021 L in Ml = 0.021 x1000=21.18 Ml ofNaOH
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To calculate moles of any 
of a subtance, we need to divide the given mass by the molar mass of the substance that is 
given as per the question. By simply putting the values of the two masses we will be left with the number of moles of the given substance.
where, n= number of moles of given substance (mol)
m= given mass of the substance (grams)
M= molar mass of the given substance (grams/mol)
