Linear programming – Q 21 LD Co in BPP Kit

Linear Programming – Q21 LD Co in BPP Kit.

Dear Sir John,

I have a question to How to construct an iso-contribution line?
Particularly if we know that: C=1.9L +6.7D
Why they have ?? If C = (1.9*6.7*1000)=12,730,

then: if L=6,700, D=0
if D=1,900, L=0.

My question , why they multiplied all thefigure in the brackets? and not added such they were supposed following the previous formule of contribution? And how they pick number 1,000?

Thank you for your answer.
Regards Jana.