To get the volume of the original solution in the dilution, we consider the dilutions that were made:
from 50 mL to 250 mL:
1/5 of original was present
from 20 mL of 250 mL:
1/12.5 of the diluted mixture was presetn
So, V=50 mL*(1/5)*(1/12.5)= 0.8 mL of original bleaching solution was present in dilution