Question 1002271
The description gives points  (p,C):
(10,1400), (15,1475), (20,1550).


Slopes among the successive points are
{{{(1475-1400)/(15-5)=15}}}
and
{{{(1550-1475)/(20-15)=15}}}
both slopes being the same, 15.
The points have a linear fit.


Picking the first data point and using point-slope form,
{{{C-1400=15(p-10)}}}
{{{C-1400=15p-150}}}
{{{C=15p-150+1400}}}
{{{highlight(C=15p+1250)}}}------Formula for cost as a function of how many attend.


The bus cost by itself is  $1250.


The last question means {{{15p+1400=1800}}}, and you want to solve for the value of p.