You will estimate the quantity of dna present in the puc18 dna prep by gel band brightness. compare your linear plasmid band in the hindiii digest lane to the λ-hindiii marker. if, for example, your linearized plasmid is half as bright as the 4, 361 bp band of the marker, then your approximate dna quantity is 90/2 or 45 ng. you loaded 10 μl of the hindiii digest onto the gel so your original concentration of dna in the digest is 45 ng/10 μl or 4.5 ng/μl. you put 10 μl of dna in your digest. all together then, you have 4.5 ng/μl x 10 μl or 45 ng of dna in the digest. i am comparing my linear hind iii digest plasmid to fragment of the λ-hindiii digest.