The description is of two points for x units and y cost , (x,y);
(10, 40) and (20, 70). These points are on a line.
The question asks for what is the y value for (25, y).
Or, trying looking at ratios from differences
Differences on the left and on the right of the columns
x y
10 40
10
30
20 70
5 d
25 y
y value wanted is 70+15, or 85.