Restriction enzymes help

[apronedsamurai]

What types of DNA fragments would result if you were to cut the plasmid shown with both restriction enzymes, EcoR1 and BamH1?

a. 1/2 20 kb, 1/4 30 kb, 1/4 10 kb
b. 1/2 20 kb, 1/2 60 kb
c. 1/2 30 kb, 1/2 50 kb
d. All 80 kb





I have identified the answer as being a.

My reasoning is this:

Distance between EcoR1 (20kb) and BamH1 (40kb) =20 kb

Distance between BamH1 (40kb) and BamH1(70kb)=30kb

Distance between BamH1 (70kb) and EcoR1 (80kb) =10kb

What I am confused by though, is the 1/2 and 1/4 component of the answer given. Can anyone give me a breakdown of that please?