Question 1206667
"partly constant and partly varies..."
What?



I waited to see if someone gives you a response for this "partly constant and partly varies" problem.  Not see any yet.  Maybe this older one may help you:  https://www.algebra.com/algebra/homework/coordinate/word/Linear_Equations_And_Systems_Word_Problems.faq.question.1143596.html


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MAYBE,....
{{{D=mV+k}}} a linear form equation


{{{system(150=m*40+k,192=m*54+k)}}}


Equate the k expressions and solve for m.


{{{k=150-40m=192-54m}}}


{{{-54m+192=-40m+150}}}

{{{-54m+40m=150-192}}}

{{{54m-40m=192-50}}}

{{{14m=42}}}

{{{7m=21}}}

{{{highlight_green(m=3)}}}


Use m to find value for k.
{{{k=150-40m}}}
{{{k=150-40*3=150-120}}}
{{{highlight_green(k=30)}}}


If this is the kind of method you expect ,  then D and V are to be related
{{{highlight(D=3V+30)}}}.



(b)
{{{D=249}}}